BIOGENESIS OF MITOCHONDRIAL HEME LYASES IN YEAST - IMPORT AND FOLDINGIN THE INTERMEMBRANE SPACE

Heme lyases are components of the mitochondrial intermembrane space fa cilitating the covalent attachment of heme to the apoforms of c-type c ytochromes. The precursors of heme lyases are synthesized in the cytos ol without the typical N-terminal mitachondrial targeting signal. Here , we have analyzed the mode of import and folding of the two heme lyas es of the yeast Saccharomyces cerevisiae, namely of cytochrome c heme lyase and of cytochrome c, heme lyase. For transport into mitochondria , both proteins use the general protein import machinery of the outer membrane, import occurred independently of a membrane potential, Delta psi, across the inner membrane and ATP in the matrix space, suggestin g that the inner membrane is not required for transport along this dir ect sorting pathway. The presence of a large folded domain in heme lya ses was utilized to study their folding in the intermembrane space. Fo rmation of this domain occurred at the same rate as import, indicating that heme lyases fold either during or immediately after their transf er across the membrane. Folding was not affected by depletion of ATP a nd Delta psi or by inhibitors of peptidylprolyl cis-trans isomerases, i.e. it does not involve homologs of known folding factors (Like Hsp60 and Hsp70). The energy derived from folding cannot be regarded as a m ajor driving force for import, since the folded domain could be import ed into mitochondria with the same efficiency as the intact protein. W e conclude that protein folding in the intermembrane space obeys princ iples different from those established for other subcellular compartme nts.