Def-6, a guanine nucleotide exchange factor for Rac1, interacts with the skeletal muscle integrin chain α7A and influences myoblast differentiation

Integrin alpha 7 beta 1 is the major laminin binding integrin receptor of muscle cells. The alpha 7 chain occurs in several splice isoforms, of which alpha 7A and alpha 7B differ in their intracellular domains only. The fact that the expression of alpha 7A and alpha 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 alpha 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 alpha 7A subunit. The specificity of the Def-6-alpha 7A interaction has been shown by direct yeast two-hybrid binding assays and coprecipitation experiments. This is the first description of an alpha 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 alpha 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 alpha 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 alpha 7A chain guiding this regulation in a spatio-temporal manner.