Polymyxin B was used as a probe to label anionic phospholipids in skeletal muscle plasma membrane. This antibiotic produces muscle surface membrane lesions that can be identified in both thin sections and freeze-fracture replicas. The membrane perturbations assumed a patchy distribution with a preferential localization at the level of the I band and A-I bands junction. Intramembraneous particles were also observed within the lesions. We consider the possibility that microdomains of anionic phospholipids in muscle plasma membrane may function in the binding of Ca++.
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