Protein interactions within the DGC can be categorized as occurring either within or across the plane of the membrane. The SG-SSPN subcomplex and ␤-DG are transmembrane proteins that interact laterally with one another in the sarcolemma. Dystrophin and the DGs provide a structural linkage across the sarcolemma to connect the extracellular matrix with intracellular F-actin (for a review, see Durbeej and Campbell, 2002). We found that SSPN perturbed both types of interactions in SSPN-Tg muscle. We propose that SSPNmediated aggregation of the SGs causes destabilization of ␣-DG attachment to the sarcolemma, which, in turn, leads to perturbation of basement membrane formation (Fig. 10). Taken together, our **data** support a model whereby SSPN overexpression disrupts protein interactions within and across the membrane bilayer (Fig. 10). SSPN expression caused clustering of the SGs into detergent insoluble aggregates, which were never observed in wild-type mice. Non-phenotypic  , 1 m. (B) Electron micrographs were taken at higher magnifications to visualize the basement membrane. The distance between adjacent sarcolemmas is greater in the SSPN-Tg muscle than in the non-Tg muscle (white arrows). The extracellular matrix is visualized as a tight linear structure between two neighboring sarcolemmas in the non-Tg tissue. However, the extracellular matrix is randomly placed in dense, disorganized clumps in the phenotypic SSPN-Tg muscle (black arrows). Bar, 200 nm. SSPN-Tg mice displayed only low levels of SG protein aggregation. We provide **data** to suggest that SSPN overexpression drives formation of SSPN hyper-oligomers within the sarcolemma. We propose that such abnormal SSPN structures disrupt proper SG protein interactions by trapping the SGs into insoluble protein aggregates.