07-415 Sigma-AldrichAnti-Sam 68 Antibody

p53 is one of the most important tumor suppressor genes involved in human carcinogenesis. Although downstream targets of p53 and their biologic functions in cancer cells have been extensively investigated, it is still far from the full understanding. Here, we demonstrate that Late Cornified Envelope Group I (LCE1) genes, which are located in the LCE gene clusters encoding multiple well-conserved stratum-corneum proteins, are novel downstream targets of p53. Exogenous p53 overexpression using an adenoviral vector system significantly enhanced the expression of LCE1 cluster genes. We also observed induction of LCE1 expressions by DNA damage, which was caused by treatment with adriamycin or UV irradiation in a wild-type p53-dependent manner. Concordantly, the induction of LCE1 by DNA damage was significantly attenuated by the knockdown of p53. Among predicted p53-binding sites within the LCE1 gene cluster, we confirmed one site to be a p53-enhancer sequence by reporter assays. Furthermore, we identified LCE1 to interact with protein arginine methyltransferase 5 (PRMT5). Knockdown of LCE1 by specific small interfering RNAs significantly increased the symmetric dimethylation of histone H3 arginine 8, a substrate of PRMT5, and overexpression of LCE1F remarkably decreased its methylation level. Our data suggest that LCE1 is a novel p53 downstream target that can be directly transactivated by p53 and is likely to have tumor suppressor functions through modulation of the PRMT5 activity.

The GSG (GRP33, Sam68, GLD-1) domain is a protein module found in an expanding family of RNA-binding proteins. The numerous missense mutations identified genetically in the GSG domain support its physiological role. Although the exact function of the GSG domain is not known, it has been shown to be required for RNA binding and oligomerization. Here it is shown that the Sam68 GSG domain plays a role in protein localization. We show that Sam68 concentrates into novel nuclear structures that are predominantly found in transformed cells. These Sam68 nuclear bodies (SNBs) are distinct from coiled bodies, gems, and promyelocytic nuclear bodies. Electron microscopic studies show that SNBs are distinct structures that are enriched in phosphorus and nitrogen, indicating the presence of nucleic acids. A GFP-Sam68 fusion protein had a similar localization as endogenous Sam68 in HeLa cells, diffusely nuclear with two to five SNBs. Two other GSG proteins, the Sam68-like mammalian proteins SLM-1 and SLM-2, colocalized with endogenous Sam68 in SNBs. Different GSG domain missense mutations were investigated for Sam68 protein localization. Six separate classes of cellular patterns were obtained, including exclusive SNB localization and association with microtubules. These findings demonstrate that the GSG domain is involved in protein localization and define a new compartment for Sam68, SLM-1, and SLM-2 in cancer cell lines.