Import of hybrid vesicular stomatitis G protein to the mitochondrial inner membrane

cDNA fusions were employed to construct a 35-kDa hybrid protein bearing the amino-terminal signal sequence of pre-ornithine carbamoyltransferase (pOCT), a mitochondrial matrix enzyme, fused to the carboxyl-terminal half of vesicular stomatitis virus G protein (VSV G). Following transcription-translation in vitro, the hybrid protein was imported by purified mitochondria and processed at its amino terminus by the matrix processing enzyme. The protein, however, remained anchored in the mitochondrial inner membrane, apparently in a transmembrane configuration, via the hydrophobic VSV G stop-transfer domain; a small portion (approximately 2 kDa) of the G protein fragment was accessible to protease digestion only after selective permeabilization of the mitochondrial outer membrane with digitonin, a finding consistent with localization of the extreme carboxyl-terminal cytoplasmic tail of G in the intermembrane space. The results demonstrate that the membrane-anchor domain of VSV G can function in a post-translational manner and can operate in membranes other than those derived from the endoplasmic reticulum. However, it appears to be selectively recognized as a stop-transfer signal by the translocation machinery of the mitochondrial inner, rather than outer, membrane.