FUNCTIONAL COOPERATION OF MITOCHONDRIAL PROTEIN IMPORT RECEPTORS IN YEAST

We have identified a 20 kDa yeast mitochondrial outer membrane protein (termed MAS20) which appears to function as a protein import receptor . We cloned, sequenced and physically mapped the MAS20 gene and found that the protein is homologous to the MOM19 import receptor from Neuro spora crassa. MAS20 and MOM19 contain the sequence motif F-X-K-A-L-X-V /L, which is repeated several times with minor variations in the MAS70 /MOM72 receptors. To determine how MAS20 functions together with the p reviously identified yeast receptor MAS70, we constructed yeast mutant s lacking either one or both of the receptors. Deletion of either rece ptor alone had little or no effect on fermentative growth and only par tially inhibited mitochondrial protein import in vivo. Deletion of bot h receptors was lethal. Deleting only MAS70 did not affect respiration ; deleting only MAS20 caused loss of respiration, but respiration coul d be restored by overexpressing MAS70. Import of the F1-ATPase beta-su bunit into isolated mitochondria was only partly inhibited by IgGs aga inst either MAS20 or MAS70, but both IgGs inhibited import completely. We conclude that the two receptors have overlapping specificities for mitochondrial precursor proteins and that neither receptor is by itse lf essential.