The 16 kDa subunit of vacuolar H+-ATPase is a novel sarcoglycan-interacting protein

The sarcoglycan complex in muscle consists of α-, β-, γ- and δ-sarcoglycan and is part of the larger dystrophin–glycoprotein complex (DGC), which is essential for maintaining muscle membrane integrity. Mutations in any of the four sarcoglycans cause limb-girdle muscular dystrophies (LGMD). In this report, we have identified a novel interaction between δ-sarcoglycan and the 16 kDa subunit c (16K) of vacuolar H+-ATPase. Co-expression studies in heterologous cell system revealed that 16K interacts specifically with δ-sarcoglycan and the highly related γ-sarcoglycan through the transmembrane domains. In cultured C2C12 myotubes, 16K forms a complex with sarcoglycans at the plasma membrane. Loss of sarcoglycans in the sarcoglycan-deficient BIO14.6 hamster destabilizes the DGC and alters the localization of 16K at the sarcolemma. In addition, the steady state level of β1-integrin is increased. Recent studies have shown that 16K also interacts directly with β1-integrin and our data demonstrated that sarcoglycans, 16K and β1-integrin were immunoprecipitated together in C2C12 myotubes. Since sarcoglycans have been proposed to participate in bi-directional signaling with integrins, our findings suggest that 16K might mediate the communication between sarcoglycans and integrins and play an important role in the pathogenesis of muscular dystrophy.