Import of proteins into mitochondria: a 70 kilodalton outer membrane protein with a large carboxy-terminal deletion is still transported to the outer membrane.

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Abstract
The yeast mitochondrial outer membrane contains a major 70‐kd protein which is coded by a nuclear gene. Two forms of this gene were isolated from a yeast genomic clone bank: the intact gene, and a truncated gene which had lost a large part of its 3′ end during the cloning procedure. Upon transformation into yeast, both the intact and the truncated gene are expressed; the truncated gene generates a shortened protein missing 203 amino acids from the carboxy‐terminus. This truncated polypeptide reacts with a monoclonal antibody against the authentic 70‐kd protein and is transported to the mitochondrial outer membrane. By integrative transformation, we have constructed a yeast mutant which lacks the 70‐kd protein and is unable to adapt to growth on a nonfermentable carbon source at 37 degrees C. This phenotypic lesion can be corrected by transforming the mutant with the intact, but not the truncated gene. The carboxy‐terminal sequence of 203 amino acids is thus necessary for the function of the protein, but not for its targeting to the mitochondrion.