Interaction of a Dimeric Mitochondrial Precursor with Phospholipid Vesicles: Direct Association of Each Subunit with the Membrane Is Required for Loss of Functionality

Binding of the precursor to mitochondrial aspartate aminotransferase to anionic phospholipid vesicles results in the loss of catalytic activity, apparently due to the inability of the bound protein to undergo the conformational transitions required for catalysis [A. Berezov, A. Iriarte, and M. Martinez-Carrion (1994)J. Biol. Chem.269, 22222–22229]. Light scattering and electron microscopy analysis indicate that presequence-dependent binding of the precursor leads to extensive vesicle aggregation brought about by their cross-linking through interaction of each of the two presequences of this dimeric protein with separate vesicles. To evaluate the possible contribution of this aggregation to the properties of the bound protein, we analyzed the membrane interaction of a hybrid dimer containing a single presequence peptide. This dimer still binds to vesicles but does not cause aggregation. The properties of the bound hybrid are intermediate between those of the free and bound homo-precursor dimer with only the presequence-carrying subunit showing alterations in its structural and functional properties. These results indicate that the conformational perturbation of the mature moiety of lipid-bound precursor is caused by the direct interaction of each subunit with the membrane through its own N-terminal presequence peptide.