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.

Polκ κ κ κ is one of many DNA polymerases involved in translesion DNA synthesis (TLS). It belongs to the Y-family of polymerases along with Polη η η η, Polι ι ι ι and hREV1. Unlike Polη η η η encoded by the xeroderma pigmentosum variant (XPV ) gene, Polκ κ κ κ is unable to bypass UV-induced DNA damage in vitro, but it is able to bypass benzo[a]pyrene (B[a]P)-adducted guanines accurately and efficiently. In an attempt to identify factor(s) targeting Polκ κ κ κ to its cognate DNA lesion(s), we searched for Polκ κ κ κ-interacting proteins by using the yeast two-hybrid assay. We found that Polκ κ κ κ interacts with a C-terminal region of hREV1. Polη η η η and Polι ι ι ι were also found to interact with the same region of hREV1. The interaction between Polκ κ κ κ and hREV1 was confirmed by pull-down and coimmunoprecipitation assays. The C-terminal region of hREV1 is known to interact with hREV7, a non-catalytic subunit of Polζ ζ ζ ζ that is another structurally unrelated TLS enzyme, and we show that Polκ κ κ κ and hREV7 bind to the same C-terminal region of hREV1. Thus, our results suggest that hREV1 plays a pivotal role in the multi-enzyme, multi-step process of translesion DNA synthesis.

Most mutations after DNA damage in yeast Saccharomyces cerevisiae are induced by error-prone translesion DNA synthesis employing scRev1 and DNA polymerase that consists of scRev3 and scRev7 proteins. Recently, the human REV1 (hREV1) and REV3 (hREV3) genes were identified, and their products were revealed to be involved in UV-induced mutagenesis, as observed for their yeast counterparts. Human REV7 (hREV7) was also cloned, and its product was found to interact with hREV3, but the biological function of hREV7 remained unknown. We report here the analyses of precise interactions in the human REV proteins. The interaction between hREV1 and hREV7 was identified by the yeast two-hybrid library screening using a bait of hREV7, which was confirmed by in vitro and in vivo binding assays. The homodimerization of hREV7 was also detected in the two-hybrid analysis. In addition, the precise domains for interaction between hREV7 and hREV1 or hREV3 and for hREV7 homodimerization were determined. Although hREV7 interacts with both hREV1 and hREV3, a stable complex formation of the three proteins was undetectable in vitro. These findings suggest the possibility that hREV7 might play an important role in regulating the enzymatic activities of hREV1 and hREV3 for mutagenesis in response to DNA damage.An error-free DNA replication system is required to pass accurate genetic information on to the next generation. However, various kinds of DNA damage induced by endogenous and exogenous factors impair this replication ability and cause genetic alterations, resulting in cancer predisposition (1). Cells have excellent systems for avoiding these genetic alterations by removing and repairing the damaged lesions before DNA replication for maintaining the genetic stability of the organism; these systems include base excision repair, nucleotide excision repair, mismatch repair, and recombination repair (2, 3). If a lesion on template DNA escapes these repair systems, a polymerase may stall at this point and start synthesis again downstream, resulting in a single strand gap in the DNA, which can be repaired by postreplication repair. Usually, recombination repair in postreplication repair can fix this gap without base substitution, but when this repair does not happen, DNA synthesis by a bypass formation across the lesion called translesion synthesis (TLS) 1 may take place to fill the gap. This TLS may be held in the last resort for DNA repair because mutations can be induced during this step (for reviews, see Refs. 4 -6).In budding yeast Saccharomyces cerevisiae, the scRAD30 gene, the product of which is DNA polymerase , is involved in the error-free TLS that can replicate DNA through cis-syn thymine-thymine (T-T) dimer in an error-free manner (7-10), whereas the scREV1, scREV3, and scREV7 genes are involved in the error-prone TLS that frequently induces mutations at the damaged lesions (for reviews, see . It is known that most mutations induced after UV irradiation are caused by the products of these three REV genes. scRev1 protein is a ter...

Girdin (girders of actin filaments) is a novel actin-binding Akt substrate that plays an important role in actin organization and Akt-dependent cell motility in fibroblasts.

Disrupted-In-Schizophrenia 1 (DISC1), a susceptibility gene for major psychiatric disorders, regulates neuronal migration and differentiation during mammalian brain development. Although roles for DISC1 in postnatal neurogenesis in the dentate gyrus (DG) have recently emerged, it is not known how DISC1 and its interacting proteins govern the migration, positioning, and differentiation of dentate granule cells (DGCs). Here, we report that DISC1 interacts with the actin-binding protein girdin to regulate axonal development. DGCs in girdin-deficient neonatal mice exhibit deficits in axonal sprouting in the cornu ammonis 3 region of the hippocampus. Girdin deficiency, RNA interference-mediated knockdown, and inhibition of the DISC1/girdin interaction lead to overextended migration and mispositioning of the DGCs resulting in profound cytoarchitectural disorganization of the DG. These findings identify girdin as an intrinsic factor in postnatal development of the DG and provide insights into the critical role of the DISC1/girdin interaction in postnatal neurogenesis in the DG.