the 1940s, the mitochondrion has been known to be the powerhouse
of the cell, due to its primary role in generating most cellular
energy in the form of ATP. Mitochondria are usually depicted as
static, kidney bean shaped organelles dispersed throughout the cytoplasm
(such as the cross section electron micrograph shown to the right).
However, it is now clear that mitochondria are dynamic organelles,
consisting of interconnected tubular networks constantly undergoing
rapid directed movements along microtubules combined with multiple
and balanced membrane fusion/fission reactions, even in resting
cells (click on image to right to watch a movie of mitochondrial
movement in a live cell). My lab is interested in determining the
protein machinery that both orchestrates and regulates mitochondrial
membrane dynamics, particularly fusion. We also aim to understand,
on a broader scale, the biological significance of dynamic mitochondria,
and the consequences of disrupting this process in relation to human
mitochondrial diseases. To do this, we are taking advantage of the
power of yeast genetics to specifically identify factors that control
membrane fusion. In addition, we using the fruit fly as a model
metazoan, to dissect in what cell-biological context a dynamic mitochondrial
compartment is required, an area of research that has yet to be
pursued in this model organism.
The reason for
this dynamic behaviour of mitochondria is not currently known, however,
disruption of these mitochondrial dynamics can result in neurodegenerative
diseases like dominant optic atrophy, and Charcot-Marie-Tooth type
2A disease. Previous studies have identified only a handful of factors
that are involved in regulating mitochondrial membrane dynamics.
The purpose of our research is to investigate two key questions
in this emerging field.
A) What role
do mitochondrial dynamics play for cell and tissue function?
B) What molecular machinery is required to orchestrate and regulate
mitochondrial double membrane fusion?
To advance our
understanding, we are taking advantage of recent advances in fluorescence
technology and genome-wide approaches, with the following specific
1) Fully characterize
in the model organism, Drosophila melanogaster, the tissue-specific
requirements for mitochondrial dynamics using genetics and mutant
2) Determine the substrate repertoire of Drosophila Rhomboid-7,
a mitochondrial protease that regulates mitochondrial dynamics.
3) Identify and characterize genes required specifically for mitochondrial
membrane fusion in yeast.
4) Identify genes that are synthetically lethal with yeast mitochondrial
into mitochondrial membrane dynamics came from our discovery that
a mitochondria-localized rhomboid protease regulated this process.
Rhomboids are 7-pass transmembrane (TM) domain proteins that cleave
their substrates in the lipid bilayer using a classical serine catalytic-triad
mechanism (like trypsin) and represent a new conserved family of
intramembrane proteases (see schematic to the right). Rhomboids
represent one of the most highly conserved protein families, with
members found in almost all organisms from bacteria and archaea
to humans. In Drosophila, they initially appeared to be dedicated
to regulating the activity of the epidermal growth factor receptor
(EGFR). However, our discovery of a novel and conserved subclass
of mitochondria-localized rhomboid proteases has shed new light
on the range of functions of this protein family. In yeast, the
main action of this mitochondrial enzyme is to control (by regulated
proteolysis) the action of Mgm1—itself a protein that regulates
mitochondrial membrane fusion.
In order to
get at the main issues in this emerging field, we are taking
a two-model system approach.
In the first
instance, we are using the fruit fly, Drosophila melanogaster.
We have made a fly that has a mutation in its mitochondrial rhomboid
(rhomboid-7). These flies are semi-viable. Homozygous mutant
adults only live for three days (normal flies live for about 60
days). In addition, these flies cannot walk or fly, the consequence
of a severe neuromuscular defect (click on the two movies to the
right to see the difference between a wild type fly and a rhomboid-7
mutant). We are currently characterizing the phenotype of this mutant,
by a number of genetic and biochemical approaches. It is our hope
that by studying mitochondrial function in a complex metazoan, we
will be able to understand better in what cellular context mitochondrial
membrane dynamics are required for function.
In a second
approach, we are using the yeast, Saccharomyces cerevisiae,
to identify novel factors that orchestrate and regulate mitochondrial
membrane fusion. By taking advantage of the existing yeast deletion
collection, we are both candidate testing, and adopting a high-throughput
approach. Genes that are identified in these screens will be further
characterized in yeast, and in flies (assuming they are conserved
through evolution). In addition to these yeast mating screens that
monitor mitochondrial membrane fusion, we are working with the laboratory
of Charlie Boone at the CCBR. A powerful strategy to discover the
function of genes is to identify other genes that when also mutated
result in cell death. This is referred to as synthetic lethality.
His lab has pioneered a technique to identify synthetic lethal interactions
of the entire yeast genome (called SGA-synthetic genetic array).
This approach identifies genes that are involved in the same pathway
or biochemical process. We will be doing SGA screens to look for
synthetic lethal genes of the mitochondrial rhomboid (RBD1)
and of its substrate (MGM1).
Compartments of the mitochondrion
movement in a mouse cell - (courtesy of D. Chan)
(click to play movie)
with its physiological substrate Spitz (in green) that when cleaved
is released from the membrane. The proposed serine-type catalytic
triad is shown in red.
Wild type flies
(click to play movie)
Rhomboid-7 mutant flies
to play movie)
McQuibban, G.A., Bulman, D.E . (2011) The PARLance of Parkinson disease. Autophagy, 7(7).
Shi, G., Lee, J.R., Grimes, D.A., Racacho, L., Ye, D., Yang, H., Ross, O.A., Farrer, M., McQuibban, G.A., Bulman. D,E. (2011) Functional alteration of PARL contributes to mitochondrial dysregulation in Parkinson's disease. Hum Mol Genet, 20 (10):1966-74.
McQuibban, G.A., Joza, N., Megighian, A., Scorzeto, M., Zanini, D., Reipert, S., Richter, C., Schweyen, R.J., Nowikovsky, K. (2010) A Drosophila mutant of LETM1, a candidate gene for seizures in Wolf-Hirschhorn syndrome. Hum Mol Genet . 19(6):987-1000.
Rujiviphat, J., Meglei, G., Rubinstein, J.L., McQuibban, G.A . (2009) Phospholipid association is essential for the dynamin-related protein Mgm1 to function in mitochondrial membrane fusion. J Biol Chem. 284(42):28682-6.
Meglei, G., McQuibban, G.A. (2009) The Dynamin-Related Protein Mgm1p Assembles into Oligomers and Hydrolyzes GTP To Function in Mitochondrial Membrane Fusion . Biochemistry 48(8):1774-84.
Whitworth, A.J., Lee, J.R., Ho, V.M., Flick, R., Chowdhury, R., McQuibban, G.A . (2008) Rhomboid-7 and HtrA2/Omi act in a common pathway with the Parkinson's disease factors Pink1 and Parkin . Dis Model Mech .1 (2-3):168-74.
Yu, L., Lopez, A., Anaflous, A., El Bali, B., Hamal, A., Ericson, E., Heisler, L.E., McQuibban, A., Giaever, G., Nislow, C., Boone, C., Brown, G.W., Bellaoui, M. (2008) Chemical-genetic profiling of imidazo[1,2-a]pyridines and -pyrimidines reveals target pathways conserved between yeast and human cells . PLoS Genet .4 (11):e1000284.
Yang, Y., Ouyang, Y., Yang, L., Beal, M.F., McQuibban, A., Vogel, H., Lu, B.(2008) Pink1 regulates mitochondrial dynamics through interaction with the fission/fusion machinery. Proc Natl Acad Sci U S A. 105 (19):7070-5.
G.A., Lee, J.R., Zheng, L., Juusola, M., Freeman, M. (2006) Rhomboid-7
regulates mitochondrial dynamics and contributes to Drosophila lifespan
and neuronal function. Current Biology, 16, 982-989.
Zhang, K., McQuibban, G.A., Silva,
C., Butler, G.S., Johnston, J.B., Holden, J., Clark-Lewis, I., Overall,
C.M., Power, C. (2003) HIV-induced metalloproteinase processing
of the chemokine stromal cell derived factor-1 causes neurodegeneration.
Nat Neurosci, 6, 1064-71.
McQuibban, G.A., Saurya, S., Freeman, M. (2003) Mitochondrial
membrane remodelling regulated by a conserved yeast rhomboid.
Nature 423, 537-541.
Overall, C.M., McQuibban, G.A., Clark-Lewis, I. (2002) Discovery
of chemokine substrates for matrix metalloproteinases by Exosite
Scanning: A new tool for degradomics. Biol. Chem.
McQuibban, G.A., Gong, J.H., Wong, J.P., Wallace, J.L., Clark-Lewis,
I., Overall, C.M. (2002) Matrix metalloproteinase processing
of monocyte chemoattractant proteins generates CC chemokine receptor
antagonists with anti-inflammatory properties in vivo.
Blood 100, 1160-1167.
McQuibban, G.A., Butler, G.S., Gong, J.H., Bendall. L., Power, C.,
Clark-Lewis, I., Overall, C.M. (2001) Matrix Metalloproteinase
Activity Inactivates the CXC Chemokine Stromal Cell-derived Factor-1.
J. Biol. Chem. 276, 43503-43508.
Overall, C.M., Tam, E., McQuibban, G.A., Morrison, C., Wallon, U.M.,
H.F., Roberts, C.R. (2000) Domain interactions in the gelatinase
A/TIMP-2/MT1-MMP activation complex. The ectodomain of the 44-kDa
form of membrane type-1 matrix metalloproteinase does not modulate
gelatinase A activation. J. Biol. Chem. 275, 39497-39506.
McQuibban, G.A., Gong, J.H., Tam, E.M., McCulloch, C.A., Clark-Lewis,
I., Overall, C.M. (2000) Inflammation dampened by gelatinase
A cleavage of monocyte chemoattractant protein-3. Science
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D.K., Ong, A.D., Lau, T.T., Wallon, U.M., DeClerck, Y.A., Tam, E.
(1999) Identification of the TIMP-2 binding site on the
gelatinase A hemopexin C-domain by site directed mutagenesis and
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