Cytoplasmic chaperones in precursor targeting to mitochondria: the role of MSF and hsp 70

Despite extensive study since the early 1980s, the mechanism by which newly synthesized protein precursors are unfolded in the cytoplasm and targeted correctly to the mitochondrial surface prior to translocation through the mitochondrial membranes is understood poorly. Recently, an N-ethylmaleimide (NEM)-sensitive cytoplasmic factor called mitochondrial import stimulation factor (MSF), which catalyses the ATP-dependent unfolding of precursor proteins, was described. Unlike the more general chaperone proteins of the hsp70 families, MSF not only unfolds proteins but also targets the unfolded precursor proteins to the mitochondria. Here, Mihara and Omura summarize what is known about MSF and speculate on how it, and other cytoplasmic factors, may be involved in mitochondrial import.