Identification of a novel human tankyrase through its interaction with the adaptor protein Grb14.

Tankyrase is an ankyrin repeat-containing poly(ADP-ribose) polymerase originally isolated as a binding partner for the telomeric protein TRF1, but recently identified as a mitogen-activated protein kinase substrate implicated in regulation of Golgi vesicle trafficking. In this study, a novel human tankyrase, designated tankyrase 2, was isolated in a yeast two-hybrid screen as a binding partner for the Src homology 2 domain-containing adaptor protein Grb14. Tankyrase 2 is a 130-kDa protein, which lacks the N-terminal histidine/proline/serine-rich region of tankyrase, but contains a corresponding ankyrin repeat region, sterile alpha motif module, and poly(ADP-ribose) polymerase homology domain. The TANKYRASE 2 gene localizes to chromosome 10q23.2 and is widely expressed, with mRNA transcripts particularly abundant in skeletal muscle and placenta. Upon subcellular fractionation, both Grb14 and tankyrase 2 associate with the low density microsome fraction, and association of these proteins in vivo can be detected by co-immunoprecipitation analysis. Deletion analyses implicate the N-terminal 110 amino acids of Grb14 and ankyrin repeats 10-19 of tankyrase 2 in mediating this interaction. This study supports a role for the tankyrases in cytoplasmic signal transduction pathways and suggests that vesicle trafficking may be involved in the subcellular localization or signaling function of Grb14.

Structural determinants of the interaction between the erbB2 receptor and the Src homology 2 domain of Grb7.

The Src homology 2 (SH2) domain-containing protein Grb7 and the erbB2 receptor tyrosine kinase are overexpressed in a subset of human breast cancers. They also co-immunoprecipitate from cell lysates and associate directly in vitro. Whereas the Grb7 SH2 domain binds strongly to erbB2, the SH2 domain of Grb14, a protein closely related to Grb7, does not. We have investigated the preferred binding site of Grb7 within the erbB2 intracellular domain and the SH2 domain residues that determine the high affinity of Grb7 compared with Grb14 for this site. Phosphopeptide competition and site-directed mutagenesis revealed that Tyr-1139 of erbB2 is the major binding site for the Grb7 SH2 domain, indicating an overlap in binding specificity between the Grb7 and Grb2 SH2 domains. Substituting individual amino acids in the Grb14 SH2 domain with the corresponding residues from Grb7 demonstrated that a Gln to Leu change at the betaD6 position imparted high affinity erbB2 interaction, paralleled by a marked increase in affinity for the Tyr-1139 phosphopeptide. The reverse switch at the betaD6 position abrogated Grb7 binding to erbB2. This residue therefore represents an important determinant of SH2 domain specificity within the Grb7 family.

Cloning and characterization of GRB14, a novel member of the GRB7 gene family.

Screening of a human breast epithelial cell cDNA library with the tyrosine-phosphorylated C terminus of the epidermal growth factor receptor identified a novel member of the GRB7 gene family, designated GRB14. In addition to a pleckstrin homology domain-containing central region homologous to the Caenorhabditis elegans protein F10E9.6/mig 10 and a C-terminal Src homology 2 (SH2) domain, a conserved N-terminal motif, P(S/A)IPNPFPEL, can now be included as a hallmark of this family. GRB14 mRNA was expressed at high levels in the liver, kidney, pancreas, testis, ovary, heart, and skeletal muscle. Anti-Grb14 antibodies recognized a protein of approximately 58 kDa in a restricted range of human cell lines. Among those of breast cancer origin, GRB14 expression strongly correlated with estrogen receptor positivity, and differential expression was also observed among human prostate cancer cell lines. A GST-Grb14 SH2 domain fusion protein exhibited strong binding to activated platelet-derived growth factor (PDGF) receptors (PDGFRs) in vitro, but association between Grb14 and beta-PDGFRs could not be detected in vivo. In serum-starved cells, Grb14 was phosphorylated on serine residues, which increased with PDGF, but not EGF, treatment. Grb14 is therefore a target for a PDGF-regulated serine kinase, an interaction that does not require PDGFR-Grb14 association.

Heregulin (HRG)-induced mitogenic signaling and cytotoxic activity of a HRG/PE40 ligand toxin in human breast cancer cells.

The heregulins (HRGs) are a family of growth factors that bind direction to erbB3 and erbB4 and induce tyrosine phosphorylation of erbB2 via receptor heterodimerization. Since erbB2, erbB3, and erbB4 (erbB2-4) are often overexpressed in human breast cancer cells, we produced recombinant HRGs and a HRG-based ligand toxin to investigate the signaling events triggered by HRGs and the ability of these ligands to specifically target such cells. Recombinant HRG beta 2 stimulated the tyrosine phosphorylation of erbB2-4 in ZR-75-1 human breast cancer cells. This was accompanied by the tyrosine phosphorylation of Shc and the formation of complexes between Shc and the adapter protein Grb2. Complexes were also detected between Shc and erbB2-4. However, GRb2 was detected in erbB2 and erbB4 but not erbB3 immunoprecipitates. Thus, these receptors exhibit mechanistic differences in their coupling to Ras signaling, and HRG beta 2 administration triggers multiple inputs into the Ras signaling pathway, involving receptor-Grb2, receptor-Shc, and Shc-Grb2 complexes. HRG beta 2 addition also stimulated the association of erbB3 with phosphatidylinositol-3-kinase. In accordance with the activation of key mitogenic signaling pathways, HRG beta 2 stimulated the proliferation of MCF-7 and T-47D human breast cancer cells. Moreover, when tested for the ability to stimulate cell cycle re-entry of T-47D cells arrested under serum-free conditions, HRG beta 2 was more effective than insulin, previously the most potent mitogen identified using this system. Finally, a HRG beta 2 PE40 ligand toxin was constructed and found to exhibit cytotoxic activity against human breast cancer cells overexpressing erbB3 alone or in combination with erbB4 and/or erbB2.

Characterization of the mouse thrombomodulin gene and functional analysis of the 5' flanking region in F9 teratocarcinoma cells.

The mouse thrombomodulin (TM) gene was examined and shown to be a single copy gene lacking introns. Two different clones each containing the entire mouse TM gene were isolated and the nucleotide sequence of a 1.4 kb fragment comprising the 5' untranslated region and 1.2 kb of flanking sequences was determined. The transcriptional initiation site was located 30 bp downstream from a classical TATA motif within this fragment. This site was used in BALB/c 3T3 cells constitutively expressing TM, and when TM expression was induced in F9 teratocarcinoma cells in response to retinoic acid (RA) and dibutyryl cAMP (dbcAMP). A reporter construct consisting of the 1.4 kb fragment fused to the chloramphenicol acetyl transferase (CAT) gene was used to examine promoter function in F9 cells. CAT activity was induced on exposure to RA and dbcAMP and mimicked the pattern of expression of the endogenous TM gene. Induction of CAT activity did not depend on a sequence resembling a palindromic retinoic acid/thyroid hormone response element. We conclude that the 1.4 kb fragment contains the mouse TM promoter together with elements that control the induction of TM expression in differentiating F9 cells.

Some contingencies of spelling.

This paper presents some speculation about the contingencies that might select standard spellings. The speculation is based on a new development in the teaching of spelling-the process writing approach, which lets standard spellings emerge collateral to a high frequency of reading and writing. The paper discusses this approach, contrasts it with behavior-analytic research on spelling, and suggests some new directions for this latter research based on a behavioral interpretation of the process writing approach to spelling.