Search

Ret finger protein (RFP) is a nuclear protein that is highly expressed in testis and in various tumor cell lines. RFP functions as a transcriptional repressor and associates with Enhancer of Polycomb 1 (EPC1), a member of the Polycomb group proteins, and Mi-2beta, a main component of the nucleosome remodeling and deacetylase (NuRD) complex. We show that RFP binds with PIAS (protein inhibitor of activated STAT) proteins, PIAS1, PIAS3, PIASxalpha and PIASy at their carboxyl-terminal region and is covalently modified by SUMO-1 (sumoylation). PIAS proteins enhance the sumoylation of RFP in a dose-dependent manner and induce the translocation of RFP into nuclear bodies reminiscent of the PML bodies. In addition, co-expression of PIAS proteins or SUMO-1 strengthened the transcriptional repressive activity of RFP. Finally, our immunohistochemical results show that RFP, SUMO-1 and PIASy localize in a characteristic nuclear structure juxtaposed with the inner nuclear membrane (XY body) of primary spermatocytes in mouse testis. These results demonstrate that the intracellular location and the transcriptional activity of RFP are modified by PIAS proteins which possess SUMO E3 ligase activities and suggest that they may play a co-operative role in spermatogenesis.

SNT/FRS2 is a lipid anchored docking protein that contains an amino-terminal myristylation signal, followed by a phosphotyrosine-binding (PTB) domain and a carboxy-terminal region with multiple tyrosine residues. Here we show that the SNT/FRS2 PTB domain binds to RET receptor tyrosine kinase activated by glial cell line-derived neurotrophic factor (GDNF) or multiple endocrine neoplasia (MEN) 2 mutations. Analyses by site directed-mutagenesis revealed that it binds to tyrosine 1062 in RET that is also known to be a binding site for the SHC adaptor protein. Whereas SHC bound to RET was associated with GRB2 and GAB1 proteins, SNT/FRS2 was associated with GRB2 only, suggesting that SNT/FRS2 is involved mainly in the activation of the RAS/mitogen activated protein kinase (MAPK) pathway but not the phosphatidylinositol 3-kinase (PI3-K)/AKT pathway. In addition, phosphorylated SNT/FRS2 appeared to directly complex with SHP-2 tyrosine phosphatase. These results suggest that tyrosine 1062 in RET provides a site for the interaction of multiple signaling molecules and that the balance of SHC and SNT/FRS2 binding may affect the nature of the intracellular signaling for cell proliferation, differentiation and survival induced by activated RET. Oncogene (2001) 20, 1929–1938. [ABSTRACT FROM AUTHOR]

Mi-2β is the main component of the nucleosome remodeling and deacetylase complex and plays an important role in epigenetic transcriptional repression. Here we show that the amino-terminal and carboxyl-terminal regions of Mi-2β have distinct transcriptional activities and bind to BRG1, a component of the SWI/SNF complex, and the RET finger protein (RFP), respectively. Analysis by luciferase reporter assay revealed that the amino-terminal region of Mi-2β has a strong transactivating ability, whereas its carboxyl-terminal region has transcriptional repressive activity. Co-localization and association of Mi-2, RFP, and histone deacetylase 1 suggested that these proteins cooperate in transcriptional repression. Furthermore, the functional importance of the association of Mi-2β and RFP was confirmed by using Rfp[sup -/-] fibroblasts. On the other hand, we demonstrated that Mi-2 and BRG1 were associated with each other and that the bromodomain region of BRG1 strongly suppressed transactivation by the amino-terminal region of Mi-2β. The findings that Mi-2β interacts with both transactivating and repressing proteins and directly associates with another chromatin remodeling protein, BRG1, provide new insight into the formation of multiprotein supercomplex involved in transcriptional regulation. [ABSTRACT FROM AUTHOR]

Summary: The serine/threonine kinase Akt (also called protein kinase B) is well known as an important regulator of cell survival and growth and has also been shown to be required for cell migration in different organisms. However, the mechanism by which Akt functions to promote cell migration is not understood. Here, we identify an Akt substrate, designated Girdin/APE (Akt-phosphorylation enhancer), which is an actin binding protein. Girdin expresses ubiquitously and plays a crucial role in the formation of stress fibers and lamellipodia. Akt phosphorylates serine at position 1416 in Girdin, and phosphorylated Girdin accumulates at the leading edge of migrating cells. Cells expressing mutant Girdin, in which serine 1416 was replaced with alanine, formed abnormal elongated shapes and exhibited limited migration and lamellipodia formation. These findings suggest that Girdin is essential for the integrity of the actin cytoskeleton and cell migration and provide a direct link between Akt and cell motility. [Copyright &y& Elsevier]