m-AAA protease-driven membrane dislocation allows intramembrane cleavage by rhomboid in mitochondria

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Abstract
Maturation of cytochrome c peroxidase (Ccp1) in mitochondria occurs by the subsequent action of two conserved proteases in the inner membrane: the m ‐AAA protease, an ATP‐dependent protease degrading misfolded proteins and mediating protein processing, and the rhomboid protease Pcp1, an intramembrane cleaving peptidase. Neither the determinants preventing complete proteolysis of certain substrates by the m ‐AAA protease, nor the obligatory requirement of the m ‐AAA protease for rhomboid cleavage is currently understood. Here, we describe an intimate and unexpected functional interplay of both proteases. The m ‐AAA protease mediates the ATP‐dependent membrane dislocation of Ccp1 independent of its proteolytic activity. It thereby ensures the correct positioning of Ccp1 within the membrane bilayer allowing intramembrane cleavage by rhomboid. Decreasing the hydrophobicity of the Ccp1 transmembrane segment facilitates its dislocation from the membrane and renders rhomboid cleavage m ‐AAA protease‐independent. These findings reveal for the first time a non‐proteolytic function of the m ‐AAA protease during mitochondrial biogenesis and rationalise the requirement of a preceding step for intramembrane cleavage by rhomboid.