ROLE OF THE INTERMEMBRANE-SPACE DOMAIN OF THE PREPROTEIN RECEPTOR TOM22 IN PROTEIN IMPORT INTO MITOCHONDRIA

Tom22 is an essential component of the protein translocation complex ( Tom complex) of the mitochondrial outer membrane. The N-terminal domai n of Tom22 functions as a preprotein receptor in cooperation with Tom2 0. The role of the C-terminal domain of Tom22, which is exposed to the intermembrane space (IMS), in its own assembly into the Tom complex a nd in the import of other preproteins was investigated. The C-terminal domain of Tom22 is not essential for the targeting and assembly of th is protein, as constructs lacking part or all of the IMS domain became imported into mitochondria and assembled into the Tom complex. Mutant strains of Neurospora expressing the truncated Tom22 proteins mere ge nerated by a novel procedure. These mutants displayed wild-type growth rates, in contrast to cells lacking Tom22, which are not viable. The import of proteins into the outer membrane and the IMS of isolated mut ant mitochondria was not affected. Some but not all preproteins destin ed for the matrix and inner membrane were imported less efficiently. T he reduced import was not due to impaired interaction of presequences with their specific binding site on the trans side of the outer membra ne. Rather, the IMS domain of Tom22 appears to slightly enhance the ef ficiency of the transfer of these preproteins to the import machinery of the inner membrane.