Separating mitochondrial protein assembly and endoplasmic reticulum tethering by selective coupling of Mdm10.

Lars Ellenrieder,Vivien Krüger,Nikolaus Pfanner,Richard Wagner,Chris Meisinger,Sebastian B Stiller,Oliver Mirus,Thomas Becker,Sebastian P Straub,Lars Becker,Łukasz Opaliński,Enrico Schleiff,Katharina Ebell,Nadine Flinner,Bernard Guiard,Nils Wiedemann

doi:10.1038/ncomms13021

Abstract

The endoplasmic reticulum–mitochondria encounter structure (ERMES) connects the mitochondrial outer membrane with the ER. Multiple functions have been linked to ERMES, including maintenance of mitochondrial morphology, protein assembly and phospholipid homeostasis. Since the mitochondrial distribution and morphology protein Mdm10 is present in both ERMES and the mitochondrial sorting and assembly machinery (SAM), it is unknown how the ERMES functions are connected on a molecular level. Here we report that conserved surface areas on opposite sides of the Mdm10 β-barrel interact with SAM and ERMES, respectively. We generated point mutants to separate protein assembly (SAM) from morphology and phospholipid homeostasis (ERMES). Our study reveals that the β-barrel channel of Mdm10 serves different functions. Mdm10 promotes the biogenesis of α-helical and β-barrel proteins at SAM and functions as integral membrane anchor of ERMES, demonstrating that SAM-mediated protein assembly is distinct from ER-mitochondria contact sites.

Highlights

The endoplasmic reticulum–mitochondria encounter structure (ERMES) connects the mitochondrial outer membrane with the ER
Since deletion of various loops of Mdm[10] neither affected cell growth nor the interaction of Mdm[10] with ERMES or SAM38, we focused on the membrane-integrated b-barrel domain
To study the interaction of Mdm[10] with its partners, mitochondria were lysed with the non-ionic detergent digitonin, and Mdm10-containing complexes were purified by co-immunoprecipitation

Summary

Introduction

The endoplasmic reticulum–mitochondria encounter structure (ERMES) connects the mitochondrial outer membrane with the ER. Multiple mitochondrial functions have been linked to ERMES, including biogenesis and assembly of outer membrane proteins, lipid homeostasis, membrane dynamics, mitophagy and maintenance of mitochondrial morphology[5,11,18,19,20,21,22,23,24]. Mutants of Mdm[10] disturb major ERMES-connected functions, including mitochondrial morphology, lipid homeostasis and protein assembly, yet it is open which molecular functions are performed by ERMES-bound Mdm[10] and which ones by SAM-bound Mdm[10]. Our findings reveal that the b-barrel of Mdm[10] plays different roles It forms a channel for the SAM-mediated insertion of Tom[22] into the outer membrane and serves as integral membrane anchor of ERMES at mitochondria, promoting its functions in membrane morphology and lipid transfer

Objectives

Results

Conclusion