Influence of N-terminal sequence variation on the sorting of major adenylate kinase to the mitochondrial intermembrane space in yeast

Major adenylate kinase (Aky2p) from yeast has no cleavable presequence and occurs in identical form in the mitochondrial intermembrane space (6-8 %) and in the cytoplasm (approx. 90 %). To identify the signal(s) on Aky2p that might be required for mitochondrial import, the N-terminal region was examined. The N-terminus of Aky2p can guide at least two cytoplasmic passengers, dihydrofolate reductase from mouse and UMP kinase (Ura6p) from yeast, to the intermembrane space in vivo, showing that the N-terminus harbours import information. In contrast, deletion of the eight N-terminal amino acid residues or the introduction of two compensating frameshifts into this segment does not abolish translocation into the organelle's intermembrane space. Thus internal targeting and sorting information must be present in Aky2p as well. Neither a pronounced amphiphilic alpha-helical moment nor positive charges in the N-terminal region is a necessary prerequisite for Aky2p to reach the intermembrane space. Even a surplus of negative charges in mutant N-termini does not impede basal import into the correct submitochondrial compartment. The potential to form an amphipathic alpha-helical structure of five to eight residues close to the N-terminus significantly improves import efficiency, whereas extension of this amphipathic structure, e.g. by replacing it with the homologous segment of Aky3p, a mitochondrial matrix protein from yeast, leads to misdirection of the chimaera to the matrix compartment. This shows that the topogenic N-terminal signal of Aky3p is dominant over the presumptive internal intermembrane space-targeting signal of Aky2p and argues that the sorting of wild-type Aky2p to the intermembrane space is not due to the presence in the protein of a specific sorting sequence for the intermembrane space, but rather is the consequence of being imported but not being sorted to the inner compartment. Some Aky2 mutant proteins are susceptible to proteolysis in the cytoplasm, indicating incorrect folding. They are nevertheless efficiently rescued by uptake into mitochondria, suggesting a negative correlation between folding velocity (or folding stability) and efficiency of import.