Membrane protein import in yeast mitochondria

The protein import pathway that targets proteins to the mitochondrial matri x has been extensively characterized in the past 15 years. Variations of th is import pathway account for the sorting of proteins to other compartments as well, but the insertion of integral inner membrane proteins lacking a p resequence is mediated by distinct translocation machinery. This consists o f a complex of Tim9 and Tim10, two homologous, Zn2+- binding proteins that chaperone the passage of the hydrophobic precursor across the aqueous inter membrane space. The precursor is then targeted to another, inner-membrane-b ound, complex of at least five subunits that facilitates insertion. Biochem ical and genetic experiments have identified the key components of this pro cess; we are now starting to understand the molecular mechanism. This revie w highlights recent advances in this new membrane protein insertion pathway .