The signal anchor sequence of mitochondrial Mas70p contains an oligomerization domain

pOMD29 is a mitochondrial precursor protein that contains the NH2-terminal signal anchor sequence of Mas70p fused to dihydrofolate reductase. The signal anchor mediates insertion of pOMD29 into the outer mitochondrial membrane in the Nin-Ccyto orientation. Following import in vitro, pOMD29 was chemically cross-linked, via a unique cysteine residue adjacent to the signal anchor (residue 34), to form a product that was approximately twice the size of pOMD29. The cross-linked product was a dimer of pOMD29, as judged by the following. 1) It exhibited the same charge:mass ratio as pOMD29. 2) Formation of radioactive cross-linked product containing 35S-labeled pOMD29 was stimulated by co-import with unlabeled pOMD29. 3) Co-import of pOMD29 with a modified pOMD29 that contains two copies of dihydrofolate reductase resulted in formation of the predicted homo- and heterodimers. Cross-linking of pOMD29 was unaffected by concentrations of methotrexate that lock the dihydrofolate reductase moiety into its native monomeric conformation, indicating that oligomerization was mediated by the signal anchor rather than by the cytosolic domain of pOMD29. The predicted transmembrane core of the signal anchor sequence contains structural motifs similar to those found in a variety of signal-transducing cell surface receptors that dimerize through their transmembrane segments.