Conserved substrate binding by chaperones in the bacterial periplasm and the mitochondrial intermembrane space
Résumé
Mitochondria were derived from intracellular bacteria and the mitochondrial intermembrane space is topologically equivalent to the bacterial periplasm. Both compartments contain ATP-independent chaperones involved in transport of hydrophobic membrane proteins. The mitochondrial TIM10 complex and the periplasmic chaperone SurA were examined in terms of evolutionary relation, structural similarity, substrate binding specificity and function in transporting polypeptides for insertion into membranes. The two chaperones are evolutionarily unrelated; structurally, they are also distinct both in their characteristics as determined by small angle X-ray scattering (SAXS) and in pairwise structural comparison using the distance matrix alignment (DALI Lite server). Despite their structural differences, SurA and the TIM10 complex share a common binding specificity both in pepscan assays of substrate proteins. Comprehensive analysis of the binding on a total of 1407 immobilised 13mer peptides revealed that the TIM10 complex, like SurA, does not bind hydrophobic peptides generally, but that both chaperones display selectivity for peptides rich in aromatic residues and with net positive charge. This common binding specificity was not sufficient for SurA to completely replace TIM10 in yeast cells in vivo. In yeast cells lacking TIM10, when SurA is targeted to the intermembrane space of mitochondria it binds translocating substrate proteins, but fails to complete transfer of the substrate to the translocase in the mitochondrial inner membrane. We suggest that SurA was incapable of presenting substrates effectively to the primitive TOM and TIM complexes in early mitochondria, and was replaced by the more effective small Tim chaperone.
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