Gem1 and ERMES Do Not Directly Affect Phosphatidylserine Transport from ER to Mitochondria or Mitochondrial Inheritance

Tammy T Nguyen,Mesut Bilgin,Tom A Rapoport,Daniel F Markgraf,Jae‐Yeon Choi,Mirco Junker,Dennis R Voelker,Agnieszka Lewandowska,Janet M Shaw,Christer S Ejsing

doi:10.1111/j.1600-0854.2012.01352.x

Abstract

In yeast, a protein complex termed the ER-Mitochondria Encounter Structure (ERMES) tethers mitochondria to the endoplasmic reticulum. ERMES proteins are implicated in a variety of cellular functions including phospholipid synthesis, mitochondrial protein import, mitochondrial attachment to actin, polarized mitochondrial movement into daughter cells during division, and maintenance of mitochondrial DNA (mtDNA). The mitochondrial-anchored Gem1 GTPase has been proposed to regulate ERMES functions. Here, we show that ERMES and Gem1 have no direct role in the transport of phosphatidylserine (PS) from the ER to mitochondria during the synthesis of phosphatidylethanolamine (PE), as PS to PE conversion is not affected in ERMES or gem1 mutants. In addition, we report that mitochondrial inheritance defects in ERMES mutants are a secondary consequence of mitochondrial morphology defects, arguing against a primary role for ERMES in mitochondrial association with actin and mitochondrial movement. Finally, we show that ERMES complexes are long-lived, and do not depend on the presence of Gem1. Our findings suggest that the ERMES complex may have primarily a structural role in maintaining mitochondrial morphology.

Highlights

Our findings suggest that the endoplasmic reticulum (ER)-Mitochondria Encounter Structure (ERMES) complex may have primarily a structural role in maintaining mitochondrial morphology
The small decrease observed for mdm34 and mdm10 could result from insufficient Psd1 in the mitochondria of these strains
Our results are more consistent with a model in which the ERMES complex serves a structural role in maintaining the morphological integrity of mitochondria

Summary

Introduction

Genetic studies have implicated Gem1 in mitochondrial inheritance during cell division [23] and regulation of lipid synthesis by ERMES [24]. We show that the ERMES complex or Gem1 have no direct role in the transport of PS from the ER to mitochondria during the synthesis of PE. When overexpressed in ERMES mutants, Ypt11 increased mitochondrial inheritance from 40 to 81% in mdm12 and from 59 to 89% in mmm1 cells (Figure 3B).

Results

Conclusion