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Transport of proteins to the mitochondrial intermembrane space: the ‘matrix‐targeting’ and the ‘sorting’ domains in the cytochrome c1 presequence.

We reported earlier that the yeast cytochrome c1 presequence (length: 61 amino acids) directs attached proteins to the mitochondrial intermembrane space and that it appears to contain two functional domains: a ‘matrix‐targeting’ domain, and a ‘sorting’ domain. We have now used gene manipulation together with two different in vivo import assays to map these two domains within the cytochrome c1 presequence. The ‘matrix‐targeting’ domain is contained within the N‐terminal 16 residues (or less); by itself, it directs attached proteins to the matrix. The ‘sorting’ domain extends into the C‐terminal 13 residues of the presequence; while it does not mediate intracellular protein transport by itself, it acts together with the preceding ‘matrix‐targeting’ sequence in sorting attached proteins into the intermembrane space. On replacing the authentic ‘matrix‐targeting’ sequence with artificial sequences of different lengths we found that sorting of proteins between the outer membrane and the intermembrane space is not exclusively determined by the length of the N‐terminal ‘matrix‐targeting’ sequence.