A 42K outer-membrane protein is a component of the yeast mitochondrial protein import site.

An engineered precursor protein that sticks in the import site of isolated yeast mitochondria can be specifically photo-crosslinked to a mitochondrial outer-membrane protein of relative molecular mass 42,000 (42K). This protein (termed import-site protein 42 or ISP 42) is exposed on the mitochondrial surface; antibodies against it block protein import into mitochondria. ISP 42 is the first identified component of the putative transmembrane machinery that imports proteins into mitochondria.