Integration of porin synthesized in vitro into outer mitochondrial membranes

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Abstract
Porin, an intrinsic protein of water mitochondrial membranes of rat liver, was synthesized in vitro in a cell-free translation system with rat liver RNA. The apparent molecular mass of porin synthesized in vitro was the same as that of its mature form (34 kDa). This porin was post-translationally intergrated into the outer membrane of rat liver mitochondria when the cell-free translation products were incubated with mitochonadria at 30.degree. C even in the presence of a protonophore (carbonyl cyanide m-chlorophenylhydrazone). Therefore, the integration of porin seemed to proceed energy-independently as reported by Freitag et al. [(1982) Eur. J. Biochem. 126, 197-202]. Its integration seemed, however, to require the participation of the inner membrane, since porin was not integrated when isolated outer mitochondrial membranes alone were incubated with the translation products. Porin in the cell-free translation products bound to the outside of the outer mitochondrial membrane when incubated with intact mitochondria at 0.degree. C for 5 min. When the incubation period at 0.degree. C was prolonged to 60 min, this porin was found in the inner membrane fraction, which contained monoamine oxidase, suggesting that porin might bind to a specific site on the outer membrane in contact or fused with the inner membrane (a so-called OM-IM site). This porin bound to the OM-IM site was integrated into the outer membrane when the membrane fraction was incubated 30.degree. C for 60 min. These observations suggest that porin bound to the outside of the outer mitochondrial membrane is integrated into the outer membrane at OM-IM site by some temperature-dependent process(es).