Immunological characterization of T-type voltage-dependent calcium channel CaV3.1 (alpha 1G) and CaV3.3 (alpha 1I) isoforms reveal differences in their localization, expression, and neural development.

Low voltage-activated calcium channels (LVAs; "T-type") modulate normal neuronal electrophysiological properties such as neuronal pacemaker activity and rebound burst firing, and may be important anti-epileptic targets. Proteomic analyses of available alpha 1G/Ca(V)3.1 and alpha 1I/Ca(V)3.3 sequences suggest numerous potential isoforms, with specific alpha 1G/Ca(V)3.1 or alpha 1I/Ca(V)3.3 domains postulated to be conserved among isoforms of each T-type channel subtype. This information was used to generate affinity-purified anti-peptide antibodies against sequences unique to alpha 1G/Ca(V)3.1 or alpha 1I/Ca(V)3.3, and these antibodies were used to compare and contrast alpha 1G/Ca(V)3.1 and alpha 1I/Ca(V)3.3 protein expression by western blotting and immunohistochemistry. Each antibody reacted with appropriately sized recombinant protein in HEK-293 cells. Regional and developmental differences in alpha 1G/Ca(V)3.1 and alpha 1I/Ca(V)3.3 protein expression were observed when the antibodies were used to probe regional brain dissections prepared from perinatal mice and adult rodents and humans. Mouse forebrain alpha 1G/Ca(V)3.1 (approximately 240 kDa) was smaller than cerebellar (approximately 260 kDa) alpha 1G/Ca(V)3.1, and expression of both proteins increased during perinatal development. In contrast, mouse midbrain and diencephalic tissues evidenced an alpha 1I/Ca(V)3.3 immunoreactive doublet (approximately 230 kDa and approximately 190 kDa), whereas other brain regions only expressed the small alpha 1I/Ca(V)3.3 isoform. A unique large alpha 1I/Ca(V)3.3 isoform (approximately 260 kDa) was expressed at birth and eventually decreased, concomitant with the appearance and gradual increase of the small alpha 1I/Ca(V)3.3 isoform. Immunohistochemistry supported the conclusion that LVAs are expressed in a regional manner, as cerebellum strongly expressed alpha 1G/Ca(V)3.1, and olfactory bulb and midbrain contained robust alpha 1I/Ca(V)3.3 immunoreactivity. Finally, strong alpha 1I/Ca(V)3.3, but not alpha 1G/Ca(V)3.1, immunoreactivity was observed in brain and spinal cord by embryonic day 14 in situ. Taken together, these data provide an anatomical and biochemical basis for interpreting LVA heterogeneity and offer evidence of developmental regulation of LVA isoform expression.

The Sos1-Rac1 signaling. Possible involvement of a vacuolar H(+)-ATPase E subunit.

We have purified and identified a 32-kDa protein interacting with the Dbl oncogene homology domain of mSos1(Sos-DH) from rat brains by glutathione S-transferase-Sos-DH affinity chromatography. Peptide sequencing revealed that the protein is identical to a positive regulatory E subunit (V-ATPase E) of a vacuolar H(+)-ATPase, which is responsible for acidification of endosome and alkalinization of intracellular pH. The interaction between V-ATPase E and Sos-DH was confirmed by yeast two-hybrid assay. A coimmunoprecipitation assay demonstrated that a V-ATPase E protein physiologically bound to mSos1, and the protein was colocalized with mSos1 in the cytoplasm, as determined by immunohistochemistry. mSos1 was found in the early endosome fraction together with V-ATPase E and Rac1, suggesting the functional involvement of mSos1/V-ATPase E complexes in the Rac1 activity at endosomes. Overexpression of V-ATPase E in COS cells enhanced the ability of mSos1 to promote the guanine nucleotide exchange activity for Rac1 and stimulated the kinase activity of Jun kinase, a downstream target of Rac1. Thus, the data indicate that V-ATPase E may participate in the regulation of the mSos1-dependent Rac1 signaling pathway involved in growth factor receptor-mediated cell growth control.

Biochemical and anatomical evidence for specialized voltage-dependent calcium channel gamma isoform expression in the epileptic and ataxic mouse, stargazer.

Inherited forms of ataxia and absence seizures in mice have been linked to defects in voltage-dependent calcium channel subunits. However, a correlation between the sites of neuronal dysfunction and the impact of the primary lesion upon calcium channel subunit expression or function has not been clearly established. For example, the mutation in stargazer mice has pleiotropic consequences including synaptic alterations in cerebellar granule cells, hippocampal CA3/mossy fibers, and cortical neurons in layer V that, presumably, lead to ataxia and seizures. Genetic analysis of stargazer mice determined that the defective gene encodes a protein expressed in brain (gamma2) with limited homology to the skeletal muscle L-type calcium channel gamma1 subunit. Although additional gamma isoforms have been subsequently identified primarily in neural tissue, little was known about the proteins they encode. Therefore, this study explored the distribution and biochemical properties of gamma2 and other gamma isoforms in wild-type and stargazer brain. We cloned human gamma2, gamma3, and gamma4 isoforms, produced specific anti-peptide antibodies to gamma isoforms and characterized both heterologously expressed and endogenous gamma. We identified regional specificity in the expression of gamma isoforms by western analysis and immunohistochemistry. We report for the first time that the mutation in the stargazer gene resulted in the loss of gamma2 protein. Furthermore, no compensatory changes in the expression of gamma3 or gamma4 protein were evident in stargazer brain. In contrast to other voltage-dependent calcium channel subunits, gamma immunostaining was striking in that it was primarily detected in regions highly enriched in excitatory glutamatergic synapses and faintly detected in cell bodies, suggesting a role for gamma in synaptic functions. Sites of known synaptic dysfunction in stargazer (the hippocampal CA3 region, dentate gyrus, and cerebellar molecular layer) were revealed as relying primarily upon gamma2, as total gamma isoform expression was dramatically decreased in these regions. Electron microscopy localized anti-gamma antibody immunostaining to dendritic structures of hippocampal mossy fiber synapses, with enrichment at postsynaptic densities. To assess the association of native gamma with voltage-dependent calcium channel or alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptor subunits, gamma isoforms (gamma2, gamma3 and gamma4) were detergent solubilized from mouse forebrain. Antibodies against a highly conserved C-terminal epitope present in gamma2, gamma3 and gamma4 immunoprecipitated voltage-dependent calcium channel subunits (alpha1B), providing the first in vivo evidence that gamma and voltage-dependent calcium channels form stable complexes. Furthermore, both anti-gamma2 antibodies and anti-alpha1B antibodies independently immunoprecipitated the AMPA receptor subunit, GluR1, from mouse forebrain homogenates. In summary, loss of gamma2 immunoreactivity in stargazer is precisely localized so as to contribute to previously characterized synaptic defects. The data in this paper provide compelling evidence that gamma isoforms form complexes in vivo with voltage-dependent calcium channels as well as AMPA receptors, are selectively and differentially expressed in neuronal processes, and localize primarily to dendritic structures in the hippocampal mossy fiber region.

Structure of the N-terminal domain of Yersinia pestis YopH at 2.0 A resolution.

Yersinia pestis, the causative agent of bubonic plague, injects effector proteins into the cytosol of mammalian cells that enable the bacterium to evade the immune response of the infected organism by interfering with eukaryotic signal transduction pathways. YopH is a modular effector composed of a C-terminal protein tyrosine phosphatase (PTPase) domain and a multifunctional N-terminal domain that not only orchestrates the secretion and translocation of YopH into eukaryotic cells but also binds tyrosine-phosphorylated target proteins to mediate substrate recognition. The crystal structure of the N-terminal domain of YopH (YopH(N); residues 1-130) has been determined at 2.0 A resolution. The amino-acid sequences that target YopH for secretion from the bacterium and translocation into eukaryotic cells form integral parts of this compactly folded domain. The structure of YopH(N) bears no resemblance to eukaryotic phosphotyrosine-binding domains, nor is it reminiscent of any known fold. Residues that have been implicated in phosphotyrosine-dependent protein binding are clustered together on one face of YopH(N), but the structure does not suggest a mechanism for protein-phosphotyrosine recognition.

Identification and characterization of SEB, a novel protein that binds to the acute undifferentiated leukemia-associated protein SET.

SET, the translocation breakpoint-encoded protein in acute undifferentiated leukemia (AUL), is a 39-kDa nuclear phosphoprotein and has an inhibitory activity for protein phosphatase 2A (PP2A). SET is fused to a putative oncoprotein, CAN/NUP214, in AUL and is thought to play a key role in leukemogenesis by its nuclear localization, protein-protein interactions and PP2A inhibitory activity. Here, we describe the isolation and characterization of a novel cDNA encoding a protein with 1542 amino-acid residues that specifically interacts in a yeast two-hybrid system as well as in human cells with SET. This new protein, which we name SEB (SET-binding protein), is identified as a 170-kDa protein by immunoprecipitation with a specific antibody and is localized predominantly in the nucleus. SEB1238--1434 is determined as a SET-binding region that specifically binds to SET182--223. SEB also has an oncoprotein Ski homologous region (amino acids 654--858), six PEST sequences and three sequential PPLPPPPP repeats at the C-terminus. SEB mRNA is expressed ubiquitously in all human adult tissues and cells examined. The SEB gene locus is assigned to the chromosome 18q21.1 that contains candidate tumor suppressor genes associated with deletions in cancer and leukemia. Although the function of SEB is not known, we propose that SEB plays a key role in the mechanism of SET-related leukemogenesis and tumorigenesis, perhaps by suppressing SET function or by regulating the transforming activity of Ski in the nucleus.

Tobacco etch virus protease: mechanism of autolysis and rational design of stable mutants with wild-type catalytic proficiency.

Because of its stringent sequence specificity, the catalytic domain of the nuclear inclusion protease from tobacco etch virus (TEV) is a useful reagent for cleaving genetically engineered fusion proteins. However, a serious drawback of TEV protease is that it readily cleaves itself at a specific site to generate a truncated enzyme with greatly diminished activity. The rate of autoinactivation is proportional to the concentration of TEV protease, implying a bimolecular reaction mechanism. Yet, a catalytically active protease was unable to convert a catalytically inactive protease into the truncated form. Adding increasing concentrations of the catalytically inactive protease to a fixed amount of the wild-type enzyme accelerated its rate of autoinactivation. Taken together, these results suggest that autoinactivation of TEV protease may be an intramolecular reaction that is facilitated by an allosteric interaction between protease molecules. In an effort to create a more stable protease, we made amino acid substitutions in the P2 and P1' positions of the internal cleavage site and assessed their impact on the enzyme's stability and catalytic activity. One of the P1' mutants, S219V, was not only far more stable than the wild-type protease (approximately 100-fold), but also a more efficient catalyst.

Comparison of the substrate specificity of the human T-cell leukemia virus and human immunodeficiency virus proteinases.

Human T-cell leukemia virus type-1 (HTLV-1) is associated with a number of human diseases. Based on the therapeutic success of human immunodeficiency virus type 1 (HIV-1) PR inhibitors, the proteinase (PR) of HTLV-1 is a potential target for chemotherapy. To facilitate the design of potent inhibitors, the subsite specificity of HTLV-1 PR was characterized and compared to that of HIV-1 PR. Two sets of substrates were used that contained single amino-acid substitutions in peptides representing naturally occurring cleavage sites in HIV-1 and HTLV-1. The original HIV-1 matrix/capsid cleavage site substrate and most of its substituted peptides were not hydrolyzed by the HTLV-1 enzyme, except for those with hydrophobic residues at the P4 and P2 positions. On the other hand, most of the peptides representing the HTLV-1 capsid/nucleocapsid cleavage site were substrates of both enzymes. A large difference in the specificity of HTLV-1 and HIV-1 proteinases was demonstrated by kinetic measurements, particularly with regard to the S4 and S2 subsites, whereas the S1 subsite appeared to be more conserved. A molecular model of the HTLV-1 PR in complex with this substrate was built, based on the crystal structure of the S9 mutant of Rous sarcoma virus PR, in order to understand the molecular basis of the enzyme specificity. Based on the kinetics of shortened analogs of the HTLV-1 substrate and on analysis of the modeled complex of HTLV-1 PR with substrate, the substrate binding site of the HTLV-1 PR appeared to be more extended than that of HIV-1 PR. Kinetic results also suggested that the cleavage site between the capsid and nucleocapsid protein of HTLV-1 is evolutionarily optimized for rapid hydrolysis.

Cloning of the bovine leukemia virus proteinase in Escherichia coli and comparison of its specificity to that of human T-cell leukemia virus proteinase.

The proteinase of bovine leukemia virus (BLV) was cloned into pMal-c2 vector with N-terminal or with N- as well as C-terminal flanking sequences, and expressed in fusion with maltose binding protein. The proteinase self-processed itself from the fusion protein during expression and formed inclusion bodies. The enzyme was purified from inclusion bodies by cation-exchange chromatography followed by gel filtration. Specificity of the enzyme was compared to that of human T-cell leukemia proteinase type 1. Although the two viruses belong to the same subfamily of retroviruses, the differences in their proteinase specificity, based on kinetics with oligopeptide substrates representing naturally occurring cleavage sites as well as on inhibition pattern, appear to be pronounced.

Effect of substrate residues on the P2' preference of retroviral proteinases.

The substrate sequence requirements for preference toward P2' Glu residue by human immunodeficiency virus type 1 (HIV-1) proteinase were studied in both the matrix protein/ capsid protein (MA/CA) and CA/p2 cleavage site sequence contexts. These sequences represent typical type 1 (-aromatic*Pro-) and type 2 (-hydrophobic* hydrophobic-) cleavage site sequences, respectively. While in the type 1 sequence context, the preference for P2' Glu over Ile or Gln was found to be strongly dependent on the ionic strength and the residues being outside the P2-P2' region of the substrate, it remained preferable in the type 2 substrates when typical type 1 substrate sequence residues were substituted into the outside regions. The pH profile of the specificity constants suggested a lower pH optimum for substrates having P2' Glu in contrast to those having uncharged residues, in both sequence contexts. The very low frequency of P2' Glu in naturally occurring retroviral cleavage sites of various retroviruses including equine infectious anemia virus (EIAV) and murine leukemia virus (MuLV) suggests that such a residue may not have a general regulatory role in the retroviral life cycle. In fact, unlike HIV-1 and HIV-2, EIAV and MuLV proteinases do not favor P2' Glu in either the MA/CA or CA/p2 sequence contexts.

Effect of serine and tyrosine phosphorylation on retroviral proteinase substrates.

Vimentin, a cellular substrate of HIV type 1 (HIV-1) proteinase, contains a protein kinase C (PKC) phosphorylation site at one of its cleavage sites. Peptides representing this site were synthesized in P2 Ser-phosphorylated and nonphosphorylated forms. While the nonphosphorylated peptide was a fairly good substrate of the enzyme, phosphorylation prevented hydrolysis. Phosphorylation of human recombinant vimentin by PKC prevented its processing within the head domain, where the phosphorylation occurred. Oligopeptides representing naturally occurring cleavage sites at the C-terminus of the Rous sarcoma virus integrase were assayed as substrates of the avian proteinase. Unlike the nonphosphorylated peptides, a Ser-phosphorylated peptide was not hydrolyzed by the enzyme at the Ser-Pro bond, suggesting the role of previously established phosphorylation in processing at this site. Ser-phosphorylated and Tyr-phosphorylated forms of model substrates were also tested as substrates of the HIV-1 and the avian retroviral proteinases. In contrast to the moderate effect of P4 Ser phosphorylation, phosphorylation of P1 Tyr prevented substrate hydrolysis by HIV-1 proteinase. Substrate phosphorylation had substantially smaller effects on the hydrolysis by the avian retroviral proteinase. As the active retroviral proteinase as well as various protein kinases are incorporated into mature virions, substrate phosphorylation resulting in attenuation or prevention of proteolytic processing may have important consequences in the regulation of the retroviral life cycle as well as in virus-host cell interactions.

Suppression of the poly(ADP-ribose) polymerase activity by DNA-dependent protein kinase in vitro.

It has been suggested that DNA-dependent protein kinase (DNA-PK) is a central component of DNA double-strand-break repair. The mechanism of DNA-PK action, however, has not been fully understood. Poly(ADP-ribose) polymerase (PARP) is another nuclear enzyme which has high affinity to DNA ends. In this study, we analysed the interaction between these two enzymes. First, DNA-PK was found to suppress the PARP activity and alters the pattern of poly(ADP-ribosyl)ation. Although DNA-PK phosphorylates PARP in a DNA-dependent manner, this modification is unlikely to be responsible for the suppression of PARP activity, since this suppression occurs even in the absence of ATP. Conversely, PARP was found to ADP-ribosylate DNA-PK in vitro. However, the auto-phosphorylation activity of DNA-PK was not influenced by this modification. In a competitive electrophoretic mobility shift assay, Ku 70/80 complex, the DNA binding component of DNA-PK, was found to have higher affinity to a short fragment of DNA than does PARP. Furthermore, co-immunoprecipitation analysis suggested direct or close association between Ku and PARP. Thus, DNA-PK suppresses PARP activity, probably through direct binding and/or sequestration of DNA-ends which serve as an important stimulator for both enzymes.

N-type calcium channel/syntaxin/SNAP-25 complex probed by antibodies to II-III intracellular loop of the alpha1B subunit.

Neuronal voltage-dependent calcium channels are integral components of cellular excitation and neurosecretion. In addition to mediating the entry of calcium across the plasma membrane, both N-type and P/Q-type voltage-dependent calcium channels have been shown to form stable complexes with synaptic vesicle and presynaptic membrane proteins, indicating a structural role for the voltage-dependent calcium channels in secretion. Recently, detailed structural analyses of N-type calcium channels have identified residues amino acids 718-963 as the site in the rat alpha1B subunit that mediates binding to syntaxin, synaptosome-associated protein of 25,000 mol. wt and synaptotagmin [Sheng et al. (1996) Nature 379, 451-454]. The purpose of this study was to employ site-directed antibodies to target domains within and outside of the interaction site on the rat alpha1B to probe potential binding sites for syntaxin/SNAP-25/synaptotagmin. Our results demonstrate that both antibodies employed in this study have access to their epitopes on the alpha1B as evidenced by equivalent immunoprecipitation of native [125I]omega-conotoxin GVIA-labeled alpha1B protein from CHAPS-solubilized preparations. The N-type voltage-dependent calcium channel immunoprecipitated by Ab CW14, the antibody directed to a domain outside of the synprint site, is associated with syntaxin and SNAP-25 with the recovery of these proteins, increasing in parallel to the recovery of alpha1B. However, when we used the antibody raised to an epitope within the synprint site (Ab CW8) to immunoprecipitate N-type calcium channels, the alpha1B was depleted of more than 65% of syntaxin and 80% of SNAP-25 when compared to the recovery of these proteins using Ab CW14. This is the first report of a defined epitope on the alpha1B subunit II-III loop (amino acids 863-875) whose perturbation by a site-directed antibody influences the dissociation of SNAP-25 and syntaxin.

Nuclear and nucleolar targeting of human ribosomal protein S25: common features shared with HIV-1 regulatory proteins.

The nuclear and nucleolar targeting properties of human ribosomal protein S25 (RPS25) were analysed by the expression of epitope-tagged RPS25 cDNAs in Cos-1 cells. The tagged RPS25 was localized to the cell nucleus, with a strong predominance in the nucleolus. At the amino terminus of RPS25, two stretches of highly basic residues juxtapose. This configuration shares common features with the nucleolar targeting signals (NOS) of lentiviral RNA-binding transactivators, including human immunodeficiency viruses' (HIV) Rev proteins. Deletion and site-directed mutational analyses demonstrated that the first NOS-like stretch is dispensable for both nuclear and nucleolar localization of RPS25, and that the nuclear targeting signal is located within the second NOS-like stretch. It has also been suggested that a set of continuous basic residues and the total number of basic residues should be required for nucleolar targeting. Signal-mediated nuclear/nucleolar targeting was further characterized by the construction and expression of a variety of chimeric constructs, utilizing three different backbones with RPS25 cDNA fragments. Immunofluorescence analyses demonstrated a 17 residue peptide of RPS25 as a potential nuclear/nucleolar targeting signal. The identified peptide signal may belong to a putative subclass of NOS, characterized by compact structure, together with lentiviral RNA-binding transactivators.

Mos positively regulates Xe-Wee1 to lengthen the first mitotic cell cycle of Xenopus.

Several key developmental events occur in the first mitotic cell cycle of Xenopus; consequently this cycle has two gap phases and is approximately 60-75 min in length. In contrast, embryonic cycles 2-12 consist only of S and M phases and are 30 min in length. Xe-Wee1 and Mos are translated and degraded in a developmentally regulated manner. Significantly, both proteins are present in the first cell cycle. We showed previously that the expression of nondegradable Mos, during early interphase, delays the onset of M phase in the early embryonic cell cycles. Here we report that Xe-Wee1 is required for the Mos-mediated M-phase delay. We find that Xe-Wee1 tyrosine autophosphorylation positively regulates Xe-Wee1 and is only detected in the first 30 min of the first cell cycle. The level and duration of Xe-Wee1 tyrosine phosphorylation is elevated significantly when the first cell cycle is elongated with nondegradable Mos. Importantly, we show that the tyrosine phosphorylation of Xe-Wee1 is required for the Mos-mediated M-phase delay. These findings indicate that Mos positively regulates Xe-Wee1 to generate the G2 phase in the first cell cycle and establish a direct link between the MAPK signal transduction pathway and Wee1 in vertebrates.

Expression and characterization of human foamy virus proteinase.

The human foamy virus proteinase was expressed in fusion with maltose binding protein in Escherichia coli and purified. The specific activity of the fusion protein was similar to that of the processed enzyme. The kinetic constants on foamy virus cleavage site substrates were very low but comparable to those obtained with the gag-encoded avian proteinase on its own substrates. The proteinase showed preference for high ionic strength and a pH optimum of 6.6. None of the tested retroviral cleavage site peptides were substrates, however, some peptides representing cleavage sites in retrotransposons were properly processed by the enzyme.

Transport and activation of the vacuolar aspartic proteinase phytepsin in barley (Hordeum vulgare L.).

The primary translation product of barley aspartic proteinase, phytepsin (EC 3.4.23.40), consists of a signal sequence, a propart, and mature enzyme forms. Here, we describe post-translational processing and activation of phytepsin during its transport to the vacuole in roots, as detected by using metabolic labeling and immunoprecipitation. After removal of the signal sequence, the glycosylated precursor of 53 kDa (P53) was produced and further processed to polypeptides of 31 and 15 kDa (P31 + P15) and, subsequently, to polypeptides of 26 and 9 kDa (P26 + P9), 45 min and 24 h after synthesis, respectively. The processing occurred in a late-Golgi compartment or post-Golgi compartment, because brefeldin A inhibited the processing, and P53 acquired partial endoglycosidase H resistance 30 min after synthesis, whereas P15 was completely resistant. The N-glycosylation inhibitor tunicamycin had no effect on transport, but the absence of glycans on P53 accelerated the proteolytic processing. Phytepsin was also expressed in baculovirus-infected insect cells. The recombinant prophytepsin underwent autoproteolytic activation in vitro and showed enzymatic properties similar to the enzyme purified from grains. However, a comparison of the in vitro/in vivo processing sites revealed slight differences, indicating that additional proteases are needed for the completion of the maturation in vivo.

In vivo phosphorylation of poly(ADP-ribose) polymerase is independent of its activation.

Poly(ADP-ribose) polymerase (PARP) is a nuclear enzyme, which is activated by DNA strand breaks. Although PARP is known to be cleaved by the cysteine protease, caspase-3/CPP32, during apoptosis, signal cascade which regulates the PARP activity has not been fully understood. In this study, we investigated post-translational modification of PARP. We found that PARP was phosphorylated by a serine kinase in vivo. PARP was activated temporarily and extensive auto-modification occurred on PARP, possibly by the fragmented DNA during apoptosis induced by etoposide in Jurkat cells. However, the phosphorylation level was not changed for up to 6 h, after PARP cleavage began in apoptosis by the treatment with etoposide. Furthermore, we showed the presence of a PARP-associated kinase in nuclear extracts of the HTLV-I infected T-cell lines but not in uninfected T-cell lines, whereas this kinase did not inhibit the PARP activity even in the presence of ATP. Taken together, in vivo phosphorylation of PARP might be independent of the activation or cleavage of PARP.

Expression of SET, an inhibitor of protein phosphatase 2A, in renal development and Wilms' tumor.

The human gene set was originally identified as a component of the set-can fusion gene produced by a somatic translocation event in a case of acute undifferentiated leukemia. In the developing kidney, set was highly expressed in the zone of nephron morphogenesis. Recently, SET was shown to be a potent and specific inhibitor of protein phosphatase 2A, a family of major serine/threonine phosphatases involved in regulating cell proliferation and differentiation. The current study sought to define further the role of SET in the regulation of renal cell proliferation and tumorigenesis. The mRNA encoding SET was expressed at much higher levels in transformed human and rodent cell lines than in cultured renal epithelial and primary endothelial cells. Consistent with a role for SET in cell proliferation, set mRNA expression was markedly reduced in cells rendered quiescent by serum starvation, contact inhibition, or differentiation. Previous findings during renal development were extended by demonstrating that SET protein expression is also much greater in developing rat and human kidney than in fully differentiated, mature kidney. Finally, high levels of set mRNA and SET protein expression were found in Wilms' tumor, but not in renal cell carcinoma, adult polycystic kidney disease or in transitional cell carcinoma.

The novel ATM-related protein TRRAP is an essential cofactor for the c-Myc and E2F oncoproteins.

The c-Myc and E2F transcription factors are among the most potent regulators of cell cycle progression in higher eukaryotes. This report describes the isolation of a novel, highly conserved 434 kDa protein, designated TRRAP, which interacts specifically with the c-Myc N terminus and has homology to the ATM/PI3-kinase family. TRRAP also interacts specifically with the E2F-1 transactivation domain. Expression of transdominant mutants of the TRRAP protein or antisense RNA blocks c-Myc- and E1A-mediated oncogenic transformation. These data suggest that TRRAP is an essential cofactor for both the c-Myc and E1A/E2F oncogenic transcription factor pathways.

Altered expression and assembly of N-type calcium channel alpha1B and beta subunits in epileptic lethargic (lh/lh) mouse.

Voltage-dependent calcium channels (VDCC) are multisubunit complexes whose expression and targeting require the assembly of the pore-forming alpha1 with auxiliary beta and alpha2/delta subunits. The developmentally regulated expression and differential assembly of beta isoforms with the alpha1B subunit to form N-type VDCC suggested a unique role for the beta4 isoform in VDCC maturation (Vance, C. L., Begg, C. M., Lee, W.-L., Haase, H., Copeland, T. D., and McEnery, M. W. (1998) J. Biol. Chem. 273, 14495-14502). The focus of this study is the expression and assembly of alpha1B and beta isoforms in the epileptic mouse, lethargic (lh/lh), a mutant anticipated to produce a truncated beta4 subunit (Burgess, D. L., Jones, J. M., Meisler, M. H., and Noebels, J. L. (1997) Cell 88, 385-392). In this report, we demonstrate that neither full-length nor truncated beta4 protein is expressed in lh/lh mice. The absence of beta4 in lh/lh mice is associated with decreased expression of N-type VDCC in forebrain and cerebellum. The most surprising characteristic of the lh/lh mouse is increased expression of beta1b protein. This result suggests a previously unidentified cellular mechanism wherein expression of the total pool of available beta subunits is under tight metabolic regulation. As a consequence of increased beta1b expression, the beta1b is increased in its incorporation into alpha1B/beta complexes relative to wild type. Thus, in striking similarity to the population of N-type VDCC present in immature rat brain, the population of N-type VDCC present in adult lh/lh mice is characterized by the absence of beta4 with increased beta1b expression and assembly into N-type VDCC. It is intriguing to speculate that the increased excitability and susceptibility to seizures observed in the lh/lh mouse arises from the inappropriate expression of an immature population of N-type VDCC throughout neuronal development.