Abstract
The Rab family of Ras-related GTPases are part of a complex signaling circuitry in eukaryotic cells, yet we understand little about the mechanisms that underlie Rab protein participation in such signal transduction networks, or how these networks are integrated at the physiological level. Reversible protein phosphorylation is widely used by cells as a signaling mechanism. Several phospho-Rabs have been identified, however the functional consequences of the modification appear to be diverse and need to be evaluated on an individual basis. In this study we demonstrate a role for phosphorylation as a negative regulatory event for the action of the yeast Rab GTPase Sec4p in regulating polarized growth. Our data suggest that the phosphorylation of the Rab Sec4p prevents interactions with its effector, the exocyst component Sec15p, and that the inhibition may be relieved by a PP2A phosphatase complex containing the regulatory subunit Cdc55p.
Highlights
Our current view of membrane traffic is far from that of a distribution network with machinery that passively responds to the tasks of cargo collection and transport
Elucidation of the molecular mechanisms that regulate the production and consumption of transport carriers is a significant challenge in cell biology, and is central to understanding the underlying contribution of membrane traffic to topics as diverse as apico-basolateral polarity and cell proliferation, how organelles can be manipulated by pathogens to facilitate intracellular entry and residence, and the modulation of cellular morphology during development to generate cell-type specificity
We included the serine at position 10 reasoning that its proximity to the phosphorylated residues of S8 and S11 might allow it to be utilized as a bypass phosphorylation site in the case of relaxed positional preference (Figure 1A)
Summary
Our current view of membrane traffic is far from that of a distribution network with machinery that passively responds to the tasks of cargo collection and transport. The Rab family of GTPases are central players in the regulation of membrane traffic. Because phosphorylation is a general regulatory modification utilized by diverse signal transduction pathways, its presence on Rab proteins represents a possible intervention point to understand the mechanisms by which membrane traffic is coordinated with other cellular pathways. Several global studies of the yeast phosphoproteome have been performed, identifying sites of phosphorylation on three Rab proteins, Sec4p, Ypt1p and Vps21p [1,2,3,4]. The Sec4p GTPase has been identified as a multi-site phosphoprotein by two independent studies [2,5]. In this study we have investigated the consequence(s) of this modification for Sec4p function
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