Role of the N-terminal signal peptide in the membrane insertion of Aquifex aeolicus F1F0 ATP synthase c-subunit

Rotary ATPases are membrane protein complexes that couple ATP hydrolysis to ion translocation across the membrane. Overall, they are evolutionarily well conserved, but the N-terminal segments of their rotary subunits (c-subunits) possess different lengths and levels of hydrophobicity across species. By analyzing the N-terminal variability, we distinguish four phylogenetic groups of c-subunits (groups 1-4). We characterize a member of group 2, the c-subunit from Aquifexaeolicus F1F0 ATP synthase, both in native cells and in a heterologous expression system. We demonstrate that its N-terminal segment forms a signal peptide with signal recognition particle (SRP) recognition features and is obligatorily required for membrane insertion. Based on our study and on previous characterizations of c-subunits from other organisms, we propose that c-subunits follow different membrane insertion pathways.