Figure 1

SSPN interacts with the sarcoglycans and forms oligomers characteristic of tetraspanins. SSPN is a tetraspanin-like protein, with four transmembrane domains, which complexes with the DGC and UGC at the sarcolemmal membrane of skeletal muscle. Site directed mutagenesis of SSPN revealed that the N- and C-termini as well as regions of the large extracellular loop (LEL, between transmembrane domains 3 and 4) are necessary for SSPN-SSPN and SSPN-SG interactions, respectively [24]. Deletion mutagenesis, in which regions of six amino acids were removed at a time, was performed to identify regions within SSPN that are important for protein interactions (left). The N-terminus and C-terminus (green) are critical for SSPN dimer formation and the LEL is important for trimer and tetramer oligomers. SSPN-SG interactions were identified in the LEL (purple) and mutations in the LEL disrupt SSPN monomer formation, likely due to disruption of thiol bonds (orange) critical for stabilizing the structure of SSPN. Immunoblot analysis of skeletal muscle lysates from SSPN transgenic mice demonstrates that SSPN forms homo-oligomers under non-reducing (NR) conditions (middle) [24]. In reducing conditions (R), SSPN exists solely in monomeric form. Similar to all tetraspanins, SSPN forms higher ordered structures through homo-oligomerization. We propose a model whereby SSPN-SSPN oligomers form a scaffold on which the DGC and UGC complexes are assembled (right). DGC, dystrophin-glycoprotein complex; DG, α/β dystroglycan; SG, sarcoglycans; UGC, utrophin-glycoprotein complex.