Title:
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The regulation of actin cytoskeleton dynamics and membrane trafficking by ARHGAP44
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Membrane shape is dynamic, and cellular membranes adopt different curvatures during
fundamental vesicle trafficking events. This is exemplified by the concept of iterative,
geometric based sorting. The BAR (Bin/Amphiphysin/Rvs) domain protein superfamily is the
largest group of proteins responsible for generating membrane curvature. The work
presented in this thesis has examined the cellular function of the BAR domain containing
Rho GTPase-activating protein 44 (ARHGAP44; also known as RhoGAP interacting with CIP4
homologs 2 (RICH2)).
Recent studies have strongly suggested that ARHGAP44 contributes to intracellular
trafficking pathways. In support of this role, work presented here demonstrates that
ARHGAP44 functions as a component of the endosomal recycling machinery. ARHGAP44
localises to the Rab4-, Rabll- tubulovesicular recycling compartment, where it facilitates
the sorting of endocytic cargo, namely the transferrin receptor and CD317/tetherin, away
from the degradation pathway promoting their redelivery to the plasma membrane.
Based on its domain organisation, ARHGAP44 would be predicted to link the actin-based
cytoskeleton with membrane curvature. I have shown that ARHGAP44 promotes
membrane deformation in vitro and in cellulo, and further examination revealed that
ARHGAP44-driven membrane tubulation is dependent on its amino-terminal amphipathic
helix. Suppression of ARHGAP44 led to an extensive loss of F-actin and an intracellular
accumulation of actin into short disorganised bundles, which were frequently found in
close proximity to EEA1-positive early endosomes. Rescue experiments revealed that the
domain organisation of ARHGAP44 is critical for its ability to regulate endocytic recycling of
the transferrin receptor. The activity of ARHGAP44's RhoGAP domain and the hydrophobic
composition of its amino-terminal amphipathic helix and conjoined BAR domain were
indispensable for this function.
In light of these findings, the function of ARHGAP44 was examined during the highly
specialised developmental process of erythropoiesis. Data acquired using a model in vitro
erythropoiesis culture system demonstrated that ARHGAP44 regulates the intracellular
distribution and cell surface homeostasis of the transferrin receptor in maturing
erythroblasts.
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