Mitochondrial Fusion in Yeast Requires the Transmembrane GTPase Fzo1p

Abstract

Membrane fusion is required to establish the morphology and cellular distribution of the mitochondrial compartment. In Drosophila, mutations in the fuzzy onions (fzo) GTPase block a developmentally regulated mitochondrial fusion event during spermatogenesis. Here we report that the yeast orthologue of fuzzy onions, Fzo1p, plays a direct and conserved role in mitochondrial fusion. A conditional fzo1 mutation causes the mitochondrial reticulum to fragment and blocks mitochondrial fusion during yeast mating. Fzo1p is a mitochondrial integral membrane protein with its GTPase domain exposed to the cytoplasm. Point mutations that alter conserved residues in the GTPase domain do not affect Fzo1p localization but disrupt mitochondrial fusion. Suborganellar fractionation suggests that Fzo1p spans the outer and is tightly associated with the inner mitochondrial membrane. This topology may be required to coordinate the behavior of the two mitochondrial membranes during the fusion reaction. We propose that the fuzzy onions family of transmembrane GTPases act as molecular switches to regulate a key step in mitochondrial membrane docking and/or fusion.

The role of mitochondria in anchoring dynein to the cell cortex extends beyond clustering the anchor protein

“…We have previously shown that synthetic constructs that tether mitochondria to the plasma membrane can rescue the severe growth defect of fzo1-1 Δ dnm1 Δ num1 cells grown at the non-permissive temperature for the fzo1-1 temperature sensitive allele [10,24]. To further examine the mitochondrial tethering activity of PAN, we tested the ability of PAN to rescue the conditional growth defect of fzo1-1 Δ dnm1 Δ num1 cells (Figure 2(e)).…”

“…Strains W303 ( ade2–1; leu2–3; his3–11, 15; trp1–1; ura3–1; can1–100 ) [31], W303 NUM1-yEGFP::HIS , W303 Δ num1::HIS , W303 Δ dyn1::HIS, and W303 fzo1-1 Δ dnm1::HIS Δ num1::KAN [10], W303 NUM1-yEGFP::KAN [12], W303 fzo1-1 [24], and W303 DYN1-mKate::HIS , Δ kar9::NATNT2 , and W303  MMR1-AID-FLAG::HYG Δ ypt11::NATNT2 TIR1::URA ( mito AID ) [17] were described previously.…”

Mitochondrial dynamics and division in budding yeast

“…Detailed analysis of yeast Fzo1p provided a direct demonstration that this GTPase regulates mitochondrial fusion. First, loss of Fzo1p function results in the rapid fragmentation of mitochondrial tubules, as described above, because fusion is blocked but fission continues unopposed [11,12]. Second, the mitochondrial fusion that normally occurs after two haploid yeast cells mate is blocked in the absence of Fzo1p function [11].…”

“…Fission can occur within a tubule or at a branchpoint in the reticulum and is regulated by a GTPase called Dnm1p [8][9][10]. Fusion always occurs between two mitochondrial tips or between the tip and the side of a tubule, and is regulated by the transmembrane GTPase Fzo1p [11,12]. A third GTPase, called Mgm1p, localizes to the intermembrane space and plays an important, but as yet undetermined, role in mitochondrial compartment remodeling [13][14][15].…”

“…By contrast, mitochondrial networks rapidly fragment in fzo1 mutant cells because mitochondrial fusion is blocked but fission continues unopposed (Fig. 3, right) [11,12].…”