# Molecular Structure and Function of an Endoplasmic Reticulum-Mitochondrion Tether

> **NIH NIH R01** · UNIVERSITY OF CALIFORNIA LOS ANGELES · 2021 · $306,150

## Abstract

ABSTRACT
 Eukaryotic cells are characterized by their exquisite compartmentalization. Membrane-bound organelles form
highly dynamic and interconnected networks. This complexity makes a permanent crosstalk between the
organelles a necessity for the coordination of cellular functions. The tight juxtaposition of membranes from
different types of organelles is essential to the controlled exchanges of matter and information within cells and
is mediated by various organelle-tethering protein complexes. Small metabolites and messengers such as
phospholipids (PLs) and Ca2+ are exchanged at these membrane contact sites (MCSs). Understanding the
molecular mechanisms that regulate interactions between organelles will offer new insights into this
fundamental aspect of eukaryotic cell biology. Our research focuses on the Endoplasmic Reticulum-
Mitochondrion Encounter Structure (ERMES), a tether identified in the model eukaryote organism yeast, and
functioning at ER-mitochondrial junctions also named Mitochondrion-Associated Membranes (MAMs). While
many groups investigate MCSs, most of them use approaches based on genetic screens and cellular imaging
methods combined with proteomics or metabolomics. We bring to bear biochemical, biophysical and structural
methods to characterize ERMES at the level of molecular structure to understand its precise cellular functions
and mechanism of action. ERMES is composed of five subunits, but besides its subunit composition nothing
else is known about its architecture and mode of assembly at MAMs. Three of these subunits, the ER-
anchored protein Mmm1, the soluble subunit Mdm12, and the mitochondrial membrane protein Mdm34,
contain a SMP domain, a lipid-binding protein domain exclusively found in proteins located at MCSs from yeast
to humans. Using mass-spectrometry, we showed that Mdm12 preferentially binds phosphatidylcholines while
our 17-Å resolution negative-stain EM structure of the Mmm1/Mdm12 hetero-tetramer revealed that the soluble
SMP domains not only bind phospholipids but also function as specific protein scaffolds to assemble the tether.
This led us to propose a first and very rudimentary structural model for the ERMES-mediated exchange of PLs
at MAMs. The lack of a biochemically tractable system reconstituted in vitro has hindered efforts to definitely
establish the function(s) of ERMES. Here, we propose to complete the reconstitution of ERMES to characterize
its subunit stoichiometry and identify its bona-fide lipid ligands. Using purified subunits we will reconstitute the
SMP-core of ERMES on two distinctly labeled types of proteoliposomes and assess tethering and PL
exchange using fluorescence-based biophysical methods in vitro. With this system, we will also dissect the
mechanisms of ERMES regulation by the tail-anchored mitochondrial GTPase Gem1, an integral ERMES
subunit that was shown to control tether assembly and lipid exchange. Last, we will determine the structure of
ERMES by a `hybrid' approach comb...

## Key facts

- **NIH application ID:** 10248518
- **Project number:** 5R01GM120173-05
- **Recipient organization:** UNIVERSITY OF CALIFORNIA LOS ANGELES
- **Principal Investigator:** Pascal Francois Egea
- **Activity code:** R01 (R01, R21, SBIR, etc.)
- **Funding institute:** NIH
- **Fiscal year:** 2021
- **Award amount:** $306,150
- **Award type:** 5
- **Project period:** 2017-09-30 → 2023-08-31

## Primary source

NIH RePORTER: https://reporter.nih.gov/project-details/10248518

## Citation

> US National Institutes of Health, RePORTER application 10248518, Molecular Structure and Function of an Endoplasmic Reticulum-Mitochondrion Tether (5R01GM120173-05). Retrieved via AI Analytics 2026-05-22 from https://api.ai-analytics.org/grant/nih/10248518. Licensed CC0.

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