3.A.29. The Mitochondrial Inner Membrane i-AAA Protease Complex (MIMP) Familly
The mitochondrial inner membrane i-AAA protease supercomplex is required for mitochondrial inner membrane protein insertion, degradation and turnover. It is important to maintain the integrity of the mitochondrial compartment and is required both for the degradation of unassembled subunit 2 of cytochrome c oxidase (COX2) and for efficient assembly of the mitochondrial respiratory chain. It binds unfolded substrates in an ATPase-independent manner; binding of folded COX2, a physiological substrate, requires an active ATPase but when COX2 is destabilized an active ATPase is no longer necessary. Central pore mutants of Yta11 (Yme1, OSD1) have been isoated (Graef and Langer 2006; Dunn et al. 2006).
Protein complexes involved in respiration, ATP synthesis, and protein import reside in the mitochondrial inner membrane. The m-AAA protease, a conserved hetero-hexameric AAA (ATPase associated with diverse cellular activities) protease, composed of the Yta10, Yta11 and Yta12 proteins, regulates mitochondrial proteostasis by mediating protein maturation and degradation. It also recognizes and mediates the dislocation of membrane-embedded substrates, including foreign transmembrane (TM) segments. The Yta10 TMS2 domain is essential for membrane dislocation for only a subset of substrates, whereas the Yta12 TMS2 domain is essential for membrane dislocation for all tested substrates, suggesting different roles of the TM domains in these two m-AAA protease subunits. m-AAA protease-mediated membrane dislocation was impaired in the presence of a large downstream hydrophilic moiety in a membrane substrate, suggesting that the m-AAA protease cannot dislocate large hydrophilic domains across the membrane. Thus, membrane dislocation depends on the TMSs (Lee et al. 2017).