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Vol. 20, Issue 6, 1618-1628, March 15, 2009

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The WASP Homologue Las17 Activates the Novel Actin-regulatory Activity of Ysc84 to Promote Endocytosis in Yeast

Alastair S. Robertson^*, Ellen G. Allwood^*, Adam P.C. Smith^*, Fiona C. Gardiner^*, Rosaria Costa^*, Steve J. Winder, and Kathryn R. Ayscough^*

Departments of *Molecular Biology and Biotechnology, and Biomedical Sciences, University of Sheffield, Sheffield S10 2TN, United Kingdom

Submitted September 29, 2008; Revised January 7, 2009; Accepted January 9, 2009
Monitoring Editor: Fred Chang

Actin plays an essential role in many eukaryotic cellular processes, including motility, generation of polarity, and membrane trafficking. Actin function in these roles is regulated by association with proteins that affect its polymerization state, dynamics, and organization. Numerous proteins have been shown to localize with cortical patches of yeast actin during endocytosis, but the role of many of these proteins remains poorly understood. Here, we reveal that the yeast protein Ysc84 represents a new class of actin-binding proteins, conserved from yeast to humans. It contains a novel N-terminal actin-binding domain termed Ysc84 actin binding (YAB), which can bind and bundle actin filaments. Intriguingly, full-length Ysc84 alone does not bind to actin, but binding can be activated by a specific motif within the polyproline region of the yeast WASP homologue Las17. We also identify a new monomeric actin-binding site on Las17. Together, the polyproline region of Las17 and Ysc84 can promote actin polymerization. Using live cell imaging, kinetics of assembly and disassembly of proteins at the endocytic site were analyzed and reveal that loss of Ysc84 and its homologue Lsb3 decrease inward movement of vesicles consistent with a role in actin polymerization during endocytosis.

Address correspondence to: Kathryn R. Ayscough (k.ayscough{at}sheffield.ac.uk)

Abbreviations used: SH3, Src homology 3; YAB, Ysc84 actin binding.

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