A sarcoglycan-dystroglycan complex anchors Dp116 and utrophin in the peripheral nervous system.

Abstract

The dystrophin-associated membrane-integrated protein complex anchors dystrophin in the sarcolemma of striated muscles and is composed of two glycoprotein subcomplexes, the dystroglycan and the sarcoglycan (SG) complexes, and a small membrane protein termed sarcospan (SPN). The SG complex consists of four transmembrane glycoproteins, alpha-SG, beta-SG, gamma-SG and delta-SG. We found that beta-SG and delta-SG were co-expressed with epsilon-SG, a alpha-SG homolog, in the peripheral nerve, but not with alpha-SG or gamma-SG. SPN, which tightly links to the SG complex in the muscle cell membrane, was absent in the peripheral nerve. These peripheral nerve SGs were colocalized at the outermost layer of the myelin sheath of nerve fibers together with the dystroglycan complex, utrophin, and a short dystrophin isoform (Dp116). Immunocytochemical analysis using SG-deficient animals showed that a defect in beta- or delta-SG led to a great reduction of all residual SGs, but not of the other proteins, i.e., dystroglycans, Dp116 and utrophin, in the peripheral nerve. This observation suggests that the epsilon-, beta- and delta-SG molecules form a complex behaving as a single unit similar to the SG complex in muscle cells. An immunoprecipitation study indicated that the SG complex is associated with the dystroglycan complex and Dp116 or utrophin. These results demonstrated that Dp116 and utrophin are anchored to a novel membrane protein architecture, which consists of the SG and dystroglycan complexes, but not SPN, in the Schwann cell membrane.