THE COMPLEX CONTAINING ACTIN-RELATED PROTEINS ARP2 AND ARP3 IS REQUIRED FOR THE MOTILITY AND INTEGRITY OF YEAST ACTIN PATCHES

Background: Structural modeling and biochemical experiments in vitro h ave implicated a multi-protein complex containing two actin-related pr oteins, 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-base d motility, such as the leading edge of moving cells and the 'tail' of actin that forms behind the intracellular pathogen Listeria, The func tion of this complex in vivo has not been characterized, however, and the sequences of the non-actin-related subunits remain to be determine d, Results: An Arp3 homologue from the budding yeast Saccharomyces cer evisiae was found to localize to cortical actin patches, highly motile structures that concentrate at sites of polarized growth during the y east cell cycle, A conditional arp3 mutant allele inhibited cortical a ctin motility at the restrictive temperature and eventually disrupted actin patches. Most Arp3 protein is found in a multi-protein complex; Lye purified this complex and determined the sequences of each of the protein subunits using a high-accuracy mass peptide-mapping technique. The proteins found in the complex are similar to those in the Acantha moeba and human Arp2/3 complexes except that the yeast complex lacks a 40 kDa subunit, which is therefore not required for the structural in tegrity of the complex, Conclusions: The Arp2/3 protein complex is con served from yeast to man, and in yeast the complex is required in vivo for the motility and integrity of cortical actin patches, We hypothes ize that these patches may move by a Listeria-like mechanism driven by actin polymerization.