Striatin, a calmodulin-dependent scaffolding protein, directly binds caveolin-1.

Caveolins are scaffolding proteins able to collect on caveolae a large number of signalling proteins bearing a caveolin-binding motif. The proteins of the striatin family, striatin, SG2NA, and zinedin, are composed of several conserved, collinearly aligned, protein-protein association domains, among which a putative caveolin-binding domain [Castets et al. (2000) J. Biol. Chem. 275, 19970-19977]. They are associated in part with membranes. These proteins are mainly expressed within neurons and thought to act both as scaffolds and as Ca(2+)-dependent signalling proteins [Bartoli et al. (1999) J. Neurobiol. 40, 234-243]. Here, we show that (1) rat brain striatin, SG2NA and zinedin co-immunoprecipitate with caveolin-1; (2) all are pulled down by glutathione-S-transferase (GST)-caveolin-1; (3) a fragment of recombinant striatin containing the putative caveolin-binding domain binds GST-caveolin-1. Hence, it is likely that the proteins of the striatin family are addressed to membrane microdomains by their binding to caveolin, in accordance with their putative role in membrane trafficking [Baillat et al. (2001) Mol. Biol. Cell 12, 663-673].     

The significance of telomeric aggregates in the interphase nuclei of tumor cells

Telomeres are TTAGGG repetitive motifs found at the ends of vertebrate chromosomes. In humans, telomeres are protected by shelterin, a complex of six proteins (de Lange [2005] Genes Dev. 19: 2100-2110). Since (Müller [1938] Collecting Net. 13: 181-198; McClintock [1941] Genetics 26: 234-282), their function in maintaining chromosome stability has been intensively studied. This interest, especially in cancer biology, stems from the fact that telomere dysfunction is linked to genomic instability and tumorigenesis (Gisselsson et al. [2001] Proc. Natl. Acad. Sci. USA 98: 12683-12688; Deng et al. [2003] Genes Chromosomes Cancer 37: 92-97; DePinho and Polyak [2004] Nat. Genetics 36: 932-934; Meeker et al. [2004] Clin. Cancer Res. 10: 3317-3326). In the present overview, we will discuss the role of telomeres in genome stability, recent findings on three-dimensional (3D) changes of telomeres in tumor interphase nuclei, and outline future avenues of research.     

c-IAP1 cooperates with Myc by acting as a ubiquitin ligase for Mad1

c-IAP1, a member of the inhibitor of apoptosis protein (IAP) family and a RING finger ubiquitin ligase (E3), has been proposed to be an important oncogene. In many types of cancers, the levels of c-IAP1 are upregulated, which contributes positively to tumorigenesis. However, the mechanism by which c-IAP1 promotes tumorigenesis has proven elusive. Although proteins in the IAP family may function as caspase inhibitors, c-IAP1 was shown to be a poor inhibitor of caspases. Here we show that c-IAP1 catalyzes ubiquitination of Max-dimerization protein-1 (Mad1), a cellular antagonist of Myc. Ubiquitination of Mad1 by c-IAP1 accelerates its degradation by the 26S proteasome pathway, and this reduction of the Mad1 levels cooperates with Myc to promote cell proliferation. Our results demonstrate that c-IAP1 exerts its oncogenic functions by promoting the degradation of an important negative regulator in the Myc pathway.     

Identification of internal ribosome entry segment (IRES)-trans-acting factors for the Myc family of IRESs

The proto-oncogenes c-, L-, and N-myc can all be translated by the alternative method of internal ribosome entry whereby the ribosome is recruited to a complex structural element (an internal ribosome entry segment [IRES]). Ribosome recruitment is dependent upon the presence of IRES-trans-acting factors (ITAFs) that act as RNA chaperones and allow the mRNA to attain the correct conformation for the interaction of the 40S subunit. One of the major challenges for researchers in this area is to determine whether there are groups of ITAFs that regulate the IRES-mediated translation of subsets of mRNAs. We have identified four proteins, termed GRSF-1 (G-rich RNA sequence binding factor 1), YB-1 (Y-box binding protein 1), PSF (polypyrimidine tract binding protein-associated splicing factor), and its binding partner, p54nrb, that bind to the myc family of IRESs. We show that these proteins positively regulate the translation of the Myc family of oncoproteins (c-, L-, and N-Myc) in vivo and in vitro. Interestingly, synthesis from the unrelated IRESs, BAG-1 and Apaf-1, was not affected by YB-1, GRSF-1, or PSF levels in vivo, suggesting that these three ITAFs are specific to the myc IRESs. Myc proteins play a role in cell proliferation; therefore, these results have important implications regarding the control of tumorigenesis.     

Targeting protein lysine methylation and demethylation in cancers

During the last decade, we saw an explosion of studies investigating the role of lysine methylation/demethylation of histones and non-histone proteins, such as p53, NF-kappaB, and E2F1. These 'Ying-Yang' post-translational modifications are important to fine-tuning the activity of these proteins. Lysine methylation and demethylation are catalyzed by protein lysine methyltransferases (PKMTs) and protein lysine demethylases (PKDMs). PKMTs, PKDMs, and their substrates have been shown to play important roles in cancers. Although the underlying mechanisms of tumorigenesis are still largely unknown, growing evidence is starting to link aberrant regulation of methylation to tumorigenesis. This review focuses on summarizing the recent progress in understanding of the function of protein lysine methylation, and in the discovery of small molecule inhibitors for PKMTs and PKDMs. We also discuss the potential and the caveats of targeting protein lysine methylation for the treatment of cancer.     

N and C-terminal sub-regions in the c-Myc transactivation region and their joint role in creating versatility in folding and binding

The proto-oncogene c-myc governs the expression of a number of genes targeting cell growth and apoptosis, and its expression levels are distorted in many cancer forms. The current investigation presents an analysis by proteolysis, circular dichroism, fluorescence and Biacore of the folding and ligand-binding properties of the N-terminal transactivation domain (TAD) in the c-Myc protein. A c-Myc sub-region comprising residues 1-167 (Myc1-167) has been investigated that includes the unstructured c-Myc transactivation domain (TAD, residues 1-143) together with a C-terminal segment, which appears to promote increased folding. Myc1-167 is partly helical, binds both to the target proteins Myc modulator-1 (MM-1) and TATA box-binding protein (TBP), and displays the characteristics of a molten globule. Limited proteolysis divides Myc1-167 in two halves, by cleaving in a predicted linker region between two hotspot mutation regions: Myc box I (MBI) and Myc box II (MBII). The N-terminal half (Myc1-88) is unfolded and does not alone bind to target proteins, whereas the C-terminal half (Myc92-167) has a partly helical fold and specifically binds both MM-1 and TBP. Although this might suggest a bipartite organization in the c-Myc TAD, none of the N and C-terminal fragments bind target protein with as high affinity as the entire Myc1-167, or display molten globule properties. Furthermore, merely linking the MBI with the C-terminal region, in Myc38-167, is not sufficient to achieve binding and folding properties as in Myc1-167. Thus, the entire N and C-terminal regions of c-Myc TAD act in concert to achieve high specificity and affinity to two structurally and functionally orthogonal target proteins, TBP and MM-1, possibly through a mechanism involving molten globule formation. This hints towards understanding how binding of a range of targets can be accomplished to a single transactivation domain.     

Atm deficiency affects both apoptosis and proliferation to augment Myc-induced lymphomagenesis

Myc oncoproteins are commonly activated in malignancies and are sufficient to provoke many types of cancer. However, the critical mechanisms by which Myc contributes to malignant transformation are not clear. DNA damage seems to be an important initiating event in tumorigenesis. Here, we show that although Myc does not directly induce double-stranded DNA breaks, it does augment activation of the Atm/p53 DNA damage response pathway, suggesting that Atm may function as a guardian against Myc-induced transformation. Indeed, we show that Atm loss augments Myc-induced lymphomagenesis and impairs Myc-induced apoptosis, which normally harnesses Myc-driven tumorigenesis. Surprisingly, Atm loss also augments the proliferative response induced by Myc, and this augmentation is associated with enhanced suppression of the expression of the cyclin-dependent kinase inhibitor p27(Kip1). Therefore, regulation of cell proliferation and p27(Kip1) seems to be a contributing mechanism by which Atm holds tumor formation in check.