Apoptotic regulation by the Crk adapter protein mediated by interactions with Wee1 and Crm1/exportin.

The adapter protein Crk contains an SH2 domain and two SH3 domains. Through binding of particular ligands to the SH2 domain and the N-terminal SH3 domain, Crk has been implicated in a number of signaling processes, including regulation of cell growth, cell motility, and apoptosis. We report here that the C-terminal SH3 domain, never shown to bind any specific signaling molecules, contains a binding site for the nuclear export factor Crm1. We find that a mutant Crk protein, deficient in Crm1 binding, promotes apoptosis. Moreover, this nuclear export sequence mutant [NES(-) Crk] interacts strongly, through its SH2 domain, with the nuclear tyrosine kinase, Wee1. Collectively, these data suggest that a nuclear population of Crk bound to Wee1 promotes apoptotic death of mammalian cells.

C-terminal regions of the human telomerase catalytic subunit essential for in vivo enzyme activity.

Most human cancer cells are thought to acquire the ability to divide beyond the capacity of normal somatic cells through illegitimately activating the gene hTERT, which encodes the catalytic subunit of telomerase. While telomerase reverse transcriptase (TERT) is conserved in most eukaryotes, mounting evidence suggests that the C terminus of the human protein may have functions unique to higher eukaryotes. To search for domains responsible for such functions, we assayed a panel of tandem substitution mutations encompassing this region of human TERT for in vitro and in vivo functionality. We found four clusters of mutations that inactivated the biochemical and biological functions of telomerase, separated by mutations that had little or no effect on enzyme activity. We also identified a region where mutations generate catalytically active but biologically inert proteins. This C-terminal region that dissociates activities of telomerase (C-DAT) does not appear to be involved in nuclear localization or protein multimerization. Instead, it appears that the C-DAT region is involved in a step of in vivo telomere synthesis after the assembly of a catalytically active enzyme. Intriguingly, all of the described regions reside in a portion of TERT that is dispensable for cellular viability in yeast, arguing for a divergent role of the C terminus in higher eukaryotes.

Characterization of the cytotoxic activities of novel analogues of the antitumor agent, lavendamycin.

Lavendamycin is a bacterially derived quinolinedione that displays significant antimicrobial and antitumor activities. However, preclinical development of lavendamycin as an anticancer agent was halted due to the poor aqueous solubility and relatively nonspecific cytotoxic activity of this compound. In this report, we have examined the cytotoxic activities of a series of novel lavendamycin analogues. The cytotoxic activities of these compounds were evaluated in clonogenic survival assays with A549 lung carcinoma cells. Compounds bearing an amide or amine substituent at the R(3) position were the most potent inhibitors of colony formation. MB-97, the most active member of this subgroup, decreased clonogenic outgrowth by 70% at a concentration of 10 n. Treatment of A549 cells with MB-97 led to an increase in p53 protein expression and phosphorylation and a concomitant increase in the expression of the p53 target gene, p21. Exposure of p53-positive cells to MB-97 triggered cell cycle arrest in G(1) and G(2) phases but induced a selective G(2)-phase arrest in p53-negative cells. MB-97 treatment also induced a higher level of apoptosis in p53-null cells relative to their p53-positive counterparts. Finally, MB-97 showed significant cytotoxic activity in the National Cancer Institute's panel of 60 cancer cell lines and antitumor activity in vivo in hollow fiber tumorigenesis assays.

Mitochondrial localization of Reaper to promote inhibitors of apoptosis protein degradation conferred by GH3 domain-lipid interactions.

Morphological hallmarks of apoptosis result from activation of the caspase family of cysteine proteases, which are opposed by a pro-survival family of inhibitors of apoptosis proteins (IAPs). In Drosophila, disruption of IAP function by Reaper, HID, and Grim (RHG) proteins is sufficient to induce cell death. RHG proteins have been reported to localize to mitochondria, which, in the case of both Reaper and Grim proteins, is mediated by an amphipathic helical domain known as the GH3. Through direct binding, Reaper can bring the Drosophila IAP (DIAP1) to mitochondria, concomitantly promoting IAP auto-ubiquitination and destruction. Whether this localization is sufficient to induce DIAP1 auto-ubiquitination has not been reported. In this study we characterize the interaction between Reaper and the mitochondria using both Xenopus and Drosophila systems. We find that Reaper concentrates on the outer surface of mitochondria in a nonperipheral manner largely mediated by GH3-lipid interactions. Importantly, we show that mitochondrial targeting of DIAP1 alone is not sufficient for degradation and requires Reaper binding. Conversely, Reaper able to bind IAPs, but lacking a mitochondrial targeting GH3 domain (DeltaGH3 Reaper), can induce DIAP1 turnover only if DIAP1 is otherwise targeted to membranes. Surprisingly, targeting DIAP1 to the endoplasmic reticulum instead of mitochondria is partially effective in allowing DeltaGH3 Reaper to promote DIAP1 degradation, suggesting that co-localization of DIAP and Reaper at a membrane surface is critical for the induction of DIAP degradation. Collectively, these data provide a specific function for the GH3 domain in conferring protein-lipid interactions, demonstrate that both Reaper binding and mitochondrial localization are required for accelerated IAP degradation, and suggest that membrane localization per se contributes to DIAP1 auto-ubiquitination and degradation.