Def-6, a Guanine Nucleotide Exchange Factor for Rac1, Interacts with the Skeletal Muscle Integrin Chain α7A and Influences Myoblast Differentiation

Abstract

Integrin ␣7␤1 is the major laminin binding integrin receptor of muscle cells. The ␣7 chain occurs in several splice isoforms, of which ␣7A and ␣7B differ in their intracellular domains only. The fact that the expression of ␣7A and ␣7B is tightly regulated during skeletal muscle development suggests different and distinct roles for both isoforms. However, so far, functional properties and interacting proteins were described for the ␣7B chain only. Using a yeast two-hybrid screen, we have found that Def-6, a guanine nucleotide exchange factor for Rac1, binds to the intracellular domain of the ␣7A subunit. The specificity of the Def-6-␣7A interaction has been shown by direct yeast two-hybrid binding assays and coprecipitation experiments. This is the first description of an ␣7A-specific and -exclusive interaction, because Def-6 did not bind to any other tested integrin cytoplasmic domain. Interestingly, the binding of Def-6 to ␣7A was abolished, when cells were cotransfected with an Src-related kinase, which is known to phosphorylate Def-6 and stimulate its exchange activity. We found expression of Def-6 was not only restricted to T-lymphocytes as described thus far but in a more widespread manner, including different muscle tissues. In cells, Def-6 is seen in newly forming cell protrusions and focal adhesions, and its localization partially overlaps with the ␣7A integrin receptor. C2C12 myoblasts overexpressing Def-6 show a delay of Rac1 inactivation during myogenic differentiation and abnormal myotube formation. Thus, our data suggest a role for Def-6 in the fine regulation of Rac1 during myogenesis with the integrin ␣7A chain guiding this regulation in a spatio-temporal manner.

Genetically Determined Proteolytic Cleavage Modulates α7β1 Integrin Function

“…The cleaved ␣7 70-kDa fragment remains associated with the ␤1 integrin subunit as does the NH 2 -terminal half of the molecule that contains the ligand binding site. As the conformations of integrin cytoplasmic tails are dynamic and dependent on ligand binding (12,74), the cleaved ␣7␤1 complex may differ in its intracellular signaling capacity and perhaps its association with molecules that interact with ␣7␤1 cytoplasmic domains (75,76). Comparison of ␣7 integrin amino acid sequences from a variety of vertebrates demonstrates that the RRQ sequence responsible for cleavage is highly conserved among vertebrates with the exception of the mouse.…”

Overexpression and clinical significance of IBP in epithelial ovarian carcinoma

“…IBP may also work together with activated Rac1 to regulate cell morphology (29) and affect cell differentiation via its interaction with integrins (30). An intramolecular basic amino acid-rich region K328-R340 (KRREQREQRERRR) exists within the IBP molecule, thereby suggesting that this molecule may be transposed into the nucleus to regulate gene expression (31).…”

MicroRNA-486–dependent modulation of DOCK3/PTEN/AKT signaling pathways improves muscular dystrophy–associated symptoms

“…However, we also show that miR-486 represses DOCK3 protein levels in muscle, but still results in increased activated RAC1 levels. This finding may be due to the fact that other DOCK family proteins (such as DOCK1) or other muscle proteins are capable of activating RAC1 (such as Def6 or Parvb) as a means of compensating for the decreased levels of DOCK3 resulting from miR-486 overexpression (81)(82)(83). Interestingly, the DOCK family of proteins has recently been shown to directly bind to phosphoinositides, which are well-characterized regulators of PTEN/AKT signaling (84).…”