Lateral segregation of sterol and channel proteins in the mitochondrial outer membrane induced by phospholipase A2: evidence from negative-stain electron microscopy using filipin.

The channel protein in the mitochondrial outer membrane of Neurospora crassa aggregates laterally into crystalline arrays by the action of phospholipase A2. When mitochondrial outer membranes are reacted with filipin and examined by negative-stain electron microscopy, filipin-sterol complexes are found everywhere on the membranes except on the crystalline channel arrays. This suggests that the channel-rich membrane domains may have a relatively low content of accessible sterol. It is proposed that in vitro segregation of protein and lipid membrane components by phospholipase A2 may reflect a mechanism by which the endogenous enzyme organizes the native mitochondrial membrane into functional domains.