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Background: Structural modeling and biochemical experiments in vitro have implicated a multi-protein complex containing two actin-related proteins, Arp2 and Arp3, as a potential actin-filament nucleation factor. This ‘Arp2/3 complex' has been identified in Acanthamoeba and human cells and has been shown to localize to regions involved in actin-based motility, such as the leading edge of moving cells and the ‘tail' of actin that forms behind the intracellular pathogen Listeria. The function of this complex in vivo has not been characterized, however, and the sequences of the non-actin-related subunits remain to be determined.

Results: An Arp3 homologue from the budding yeast Saccharomyces cerevisiae was found to localize to cortical actin patches, highly motile structures that concentrate at sites of polarized growth during the yeast cell cycle. A conditional arp3 mutant allele inhibited cortical actin motility at the …