The Mitochondrial GTPase Gem1 Contributes to the Cell Wall Stress Response and Invasive Growth of Candida albicans.

Barbara Koch,Tricia L Lo,Peter Boag,Ana Traven,Stevan Novakovic,Timothy M Tucey

doi:10.3389/fmicb.2017.02555

Abstract

The interactions of mitochondria with the endoplasmic reticulum (ER) are crucial for maintaining proper mitochondrial morphology, function and dynamics. This enables cells to utilize their mitochondria optimally for energy production and anabolism, and it further provides for metabolic control over developmental decisions. In fungi, a key mechanism by which ER and mitochondria interact is via a membrane tether, the protein complex ERMES (ER-Mitochondria Encounter Structure). In the model yeast Saccharomyces cerevisiae, the mitochondrial GTPase Gem1 interacts with ERMES, and it has been proposed to regulate its activity. Here we report on the first characterization of Gem1 in a human fungal pathogen. We show that in Candida albicans Gem1 has a dominant role in ensuring proper mitochondrial morphology, and our data is consistent with Gem1 working with ERMES in this role. Mitochondrial respiration and steady state cellular phospholipid homeostasis are not impacted by inactivation of GEM1 in C. albicans. There are two major virulence-related consequences of disrupting mitochondrial morphology by GEM1 inactivation: C. albicans becomes hypersusceptible to cell wall stress, and is unable to grow invasively. In the gem1Δ/Δ mutant, it is specifically the invasive capacity of hyphae that is compromised, not the ability to transition from yeast to hyphal morphology, and this phenotype is shared with ERMES mutants. As a consequence of the hyphal invasion defect, the gem1Δ/Δ mutant is drastically hypovirulent in the worm infection model. Activation of the mitogen activated protein (MAP) kinase Cek1 is reduced in the gem1Δ/Δ mutant, and this function could explain both the susceptibility to cell wall stress and lack of invasive growth. This result establishes a new, respiration-independent mechanism of mitochondrial control over stress signaling and hyphal functions in C. albicans. We propose that ER-mitochondria interactions and the ER-Mitochondria Organizing Network (ERMIONE) play important roles in adaptive responses in fungi, in particular cell surface-related mechanisms that drive invasive growth and stress responsive behaviors that support fungal pathogenicity.

Highlights

Mitochondria are key energy-producing organelles, and they further constitute a platform for integration of metabolic and stress response inputs with cellular growth and developmental decisions
A protein sequence alignment uncovered a stretch of 56 amino acids located in the GTPase Domain I that is unique to C. albicans Gem1, and not found in either S. cerevisiae Gem1 nor human Miro1 or Miro2 (Figure 1B and Figure S1)
Steady state phospholipid levels were not different in the gem1 / mutant relative to control strains. While this result does not exclude a role for Gem1 in endoplasmic reticulum (ER)-mitochondria phospholipid movements, as has been suggested based on data in S. cerevisiae (Kornmann et al, 2011), it does show that there is no major disruption of phospholipid homeostasis in the C. albicans gem1 / strain under the conditions tested here

Summary

Introduction

Mitochondria are key energy-producing organelles, and they further constitute a platform for integration of metabolic and stress response inputs with cellular growth and developmental decisions To perform these important functions, mitochondria need to communicate extensively with other cellular compartments, including the nucleus, the endoplasmic reticulum (ER), the vacuole, the plasma membrane and peroxisomes (Murley and Nunnari, 2016; Eisenberg-Bord and Schuldiner, 2017a,b). To achieve this inter-organellar communication, and thereby integrate complex signals to produce growth and development outputs, eukaryotes evolved several mechanisms of organelle-toorganelle interactions These include metabolites serving as messengers, regulated targeting of proteins to multiple cellular compartments, and membrane to membrane linkages that have been termed “membrane contact sites” (Murley and Nunnari, 2016; Eisenberg-Bord and Schuldiner, 2017a,b).

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