Mgr2 is a new component of the mitochondrial presequence translocase required for efficient coupling of the TIM23 core complex to Tim21, respiratory chain complexes, and the translocase of the outer mitochondrial membrane.

Together, our studies identify a pathway promoting germ cell formation from somatic precursor cells.

The majority of preproteins destined for mitochondria carry N-terminal presequences. The presequence translocase of the inner mitochondrial membrane (TIM23 complex) plays a central role in protein sorting. Preproteins are either translocated through the TIM23 complex into the matrix or are laterally released into the inner membrane. We report that the small hydrophobic protein Mgr2 controls the lateral release of preproteins. Mgr2 interacts with preproteins in transit through the TIM23 complex. Overexpression of Mgr2 delays preprotein release, whereas a lack of Mgr2 promotes preprotein sorting into the inner membrane. Preproteins with a defective inner membrane sorting signal are translocated into the matrix in wild-type mitochondria but are released into the inner membrane in Mgr2-deficient mitochondria. We conclude that Mgr2 functions as a lateral gatekeeper of the mitochondrial presequence translocase, providing quality control for the membrane sorting of preproteins.

Organ morphogenesis largely relies on cell division and elongation, which need to be both coordinated between cells and orchestrated with cytoskeleton dynamics. However, components that bridge the biological signals and the effectors that define cell shape remain poorly described. We have addressed this issue through the functional characterisation of QUIRKY (QKY), previously isolated as being involved in the STRUBBELIG (SUB) genetic pathway that controls cell-cell communication and organ morphogenesis in Arabidopsis. QKY encodes a protein containing multiple C2 domains and transmembrane regions, and SUB encodes an atypical LRRreceptor-like kinase. We show that twisting of the gynoecium observed in qky results from the abnormal division pattern and anisotropic growth of clustered cells arranged sporadically along the gynoecium. Moreover, the cortical microtubule (CMT) network of these cells is disorganised. A cross to botero, a katanin mutant in which the normal orientation of CMTs and anisotropic cell expansion are impaired, strongly reduces silique deviation, reinforcing the hypothesis of a role for QKY in CMT-mediated cell growth anisotropy. We also show that QKY is localised at the plasma membrane and functions in a multiprotein complex that includes SUB and PAL OF QUIRKY (POQ), a previously uncharacterised PB1-domaincontaining protein that localises both at the plasma membrane and in intracellular compartments. Our data indicate that QKY and its interactors play central roles linking together cell-cell communication and cellular growth.

SummaryThe mitochondrial inner membrane harbors three protein translocases. Presequence translocase and carrier translocase are essential for importing nuclear-encoded proteins. The oxidase assembly (OXA) translocase is required for exporting mitochondrial-encoded proteins; however, different views exist about its relevance for nuclear-encoded proteins. We report that OXA plays a dual role in the biogenesis of nuclear-encoded mitochondrial proteins. First, a systematic analysis of OXA-deficient mitochondria led to an unexpected expansion of the spectrum of OXA substrates imported via the presequence pathway. Second, biogenesis of numerous metabolite carriers depends on OXA, although they are not imported by the presequence pathway. We show that OXA is crucial for the biogenesis of the Tim18-Sdh3 module of the carrier translocase. The export translocase OXA is thus required for the import of metabolite carriers by promoting assembly of the carrier translocase. We conclude that OXA is of central importance for the biogenesis of the mitochondrial inner membrane.

Background:The TIM23 machinery cooperates with partner protein complexes of the inner mitochondrial membrane to drive preprotein import. Results: The mitochondrial ADP/ATP carrier is efficiently copurified with TIM23 complexes independently of the respiratory chain. Conclusion:The ADP/ATP carrier is an integral part of the TIM23 protein network and supports preprotein import. Significance: This study reveals an unexpected link between mitochondrial protein biogenesis and metabolite transport.

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