Presequence-dependent folding ensures MrpL32 processing by them-AAA protease in mitochondria

Abstract

m-AAA proteases exert dual functions in the mitochondrial inner membrane: they mediate the processing of specific regulatory proteins and ensure protein quality control degrading misfolded polypeptides to peptides. Loss of these activities leads to neuronal cell death in several neurodegenerative disorders. However, it is unclear how the m-AAA protease chooses between specific processing and complete degradation. A central and conserved function of the m-AAA protease is the processing of the ribosomal subunit MrpL32, which regulates ribosome biogenesis and the formation of respiratory complexes. Here, we demonstrate that the formation of a tightly folded domain harbouring a conserved CxxC-X 9 -CxxC sequence motif halts degradation initiated from the N-terminus and triggers the release of mature MrpL32. Oxidative stress impairs folding of MrpL32, resulting in its degradation by the m-AAA protease and decreased mitochondrial translation. Surprisingly, MrpL32 folding depends on its mitochondrial targeting sequence. Presequence-assisted folding of MrpL32 requires the complete import of the MrpL32 precursor before maturation occurs and therefore explains the need for post-translocational processing by the m-AAA protease rather than co-translocational cleavage by the general mitochondrial processing peptidase.

Mitochondrial protein homeostasis

“…Of great importance is the processing of one of the subunits of mitochondrial ribosome, MrpL32, executed by the m-AAA protease, also a member of the AAAþ family (85,86). This processing event is necessary for the biogenesis of functional mitochondrial ribosomes and defects in this activity are directly linked to neuropathology.…”

Chaperones and Proteases of Mitochondria: From Protein Folding and Degradation to Mitophagy

“…In addition to the general PQC activity, the m-AAA protease has also been shown to participate in processing events that are required to remove import targeting signal sequences and are a prerequisite for the acquisition of a native structure after import has been completed. A typical example is the ribosomal protein MrpL32 [98]. It can be expected that the defective biogenesis of a single protein with a central function like MrpL32 leads to a major and general defect in mitochondrial activities.…”

Metalloproteases of the Inner Mitochondrial Membrane

“…Consistent with this idea, studies in yeast have determined that a certain length of the polypeptide’s unfolded region is required for substrate retention and processing by m-AAA. 55 However, molecular determinants of substrate recognition by m-AAA, as well as the identity of potential degrons, remain to be investigated.…”