Structural Basis of Membrane Protein Chaperoning through the Mitochondrial Intermembrane Space

International audienceThe exchange of metabolites between the mitochon- drial matrix and the cytosol depends on b-barrel channels in the outer membrane and a-helical carrier proteins in the inner membrane. The essential trans- locase of the inner membrane (TIM) chaperones escort these proteins through the intermembrane space, but the structural and mechanistic details remain elusive. We have used an integrated struc- tural biology approach to reveal the functional princi- ple of TIM chaperones. Multiple clamp-like binding sites hold the mitochondrial membrane proteins in a translocation-competent elongated form, thus mimicking characteristics of co-translational mem- brane insertion. The bound preprotein undergoes conformational dynamics within the chaperone bind- ing clefts, pointing to a multitude of dynamic local binding events. Mutations in these binding sites cause cell death or growth defects associated with impairment of carrier and b-barrel protein biogen- esis. Our work reveals how a single mitochondrial ‘‘transfer-chaperone’’ system is able to guide a-heli- cal and b-barrel membrane proteins in a ‘‘nascent chain-like’’ conformation through a ribosome-free compartment

Proteomic Analysis of the Yeast Mitochondrial Outer Membrane Reveals Accumulation of a Subclass of Preproteins

Mitochondria consist of four compartments–outer membrane, intermembrane space, inner membrane, and matrix—with crucial but distinct functions for numerous cellular processes. A comprehensive characterization of the proteome of an individual mitochondrial compartment has not been reported so far. We used a eukaryotic model organism, the yeast Saccharomyces cerevisiae, to determine the proteome of highly purified mitochondrial outer membranes. We obtained a coverage of ∼85% based on the known outer membrane proteins. The proteome represents a rich source for the analysis of new functions of the outer membrane, including the yeast homologue (Hfd1/Ymr110c) of the human protein causing Sjögren–Larsson syndrome. Surprisingly, a subclass of proteins known to reside in internal mitochondrial compartments were found in the outer membrane proteome. These seemingly mislocalized proteins included most top scorers of a recent genome-wide analysis for mRNAs that were targeted to mitochondria and coded for proteins of prokaryotic origin. Together with the enrichment of the precursor form of a matrix protein in the outer membrane, we conclude that the mitochondrial outer membrane not only contains resident proteins but also accumulates a conserved subclass of preproteins destined for internal mitochondrial compartments