Genetically Determined Proteolytic Cleavage Modulates α7β1 Integrin Function

The dystrophin-glycoprotein complex and the alpha 7 beta 1 integrin are trans-sarcolemmal linkage systems that connect and transduce contractile forces between muscle fibers and the extracellular matrix. alpha 7 beta 1 is the major laminin binding integrin in skeletal muscle. Different functional variants of this integrin are generated by alternative splicing and post-translational modifications such as glycosylation and ADP-ribosylation. Here we report a species-specific difference in alpha 7 chains that results from an intra-peptide proteolytic cleavage, by a serine protease, at the (603)RRQ(605) site. Site-directed mutagenesis of RRQ to GRQ prevents this cleavage. This RRQ sequence in the alpha 7 integrin chain is highly conserved among vertebrates but it is absent in mice. Protein structure modeling indicates this cleavage site is located in an open region between the alpha-propeller and thigh domains of the alpha 7 chain. Compared with the non-cleavable alpha 7 chain, the cleaved form enhances cell adhesion and spreading on laminin. Cleavage of the alpha 7 chain is elevated upon myogenic differentiation, and this cleavage may be mediated by urokinase-type plasminogen activator. These results suggest proteolytic cleavage is a novel mechanism that regulates alpha 7 integrin functions in skeletal muscle, and that the generation of such cleavage sites is another evolutionary mechanism for expanding and modifying protein functions.