Targeting of a chemically pure preprotein to mitochondria does not require the addition of a cytosolic signal recognition factor.

To analyze the role of cytosolic cofactors in mitochondrial protein targeting, we prepared a chemically pure mitochondrial preprotein. When diluted out of 7 M urea, this precursor protein was efficiently imported into mitochondria without the addition of cytosolic cofactors. Extensive prewashing of mitochondria (up to 2 M KCl) did not reduce its import. Import of the purified precursor showed the characteristics of authentic mitochondrial import including use of the receptor MOM19, requirement for a membrane potential, and proteolytic processing. When the precursor was preincubated at a low concentration of urea, cytosolic cofactors were needed to preserve its import competence. We conclude that targeting of this preprotein via the mitochondrial master receptor MOM19 does not require a cytosolic signal recognition factor; cytosolic cofactors apparently have chaperone-like functions in mitochondrial protein uptake. Moreover, we found that a cleavable presequence was sufficient to direct protein import via MOM19. Together with the cofactor-independent function of MOM19, it is thus conceivable that MOM19 functions as mitochondrial presequence receptor.