Porin is present in the plasma membrane where it is concentrated in caveolae and caveolae-related domains

Mitochondrial porin, or voltage-dependent anion channel, is a pore-forming protein first discovered in the outer mitochondrial membrane. Later investi gations have provided indications for its presence also in other cellular m embranes, including the plasma membrane, and in caveolae. This extra-mitoch ondrial localization is debated and no clear-cut conclusion has been reache d up to now. In this work, we used biochemical and electrophysiological tec hniques to detect and characterize porin within isolated caveolae and caveo lae-like domains (low density Triton-insoluble fractions). A new procedure was used to isolate porin from plasma membrane. The outer surface of cultur ed CEM cells was biotinylated by an impermeable reagent. Low density Triton -insoluble fractions were prepared from the labeled cells and used as start ing material to purify a biotinylated protein with the same electrophoretic mobility and immunoreactivity of mitochondrial porin. In planar bilayers, the porin from these sources formed slightly anion-selective pores with pro perties indistinguishable from those of mitochondrial porin. This work thus provides a strong indication of the presence of porin in the plasma membra ne, and specifically in caveolae and caveolae-like domains.