p53-cofactor JMY is a multifunctional actin nucleation factor

Many cellular structures are assembled from networks of actin filaments and the architecture of these networks depends on the mechanism by which the filaments are formed. Several classes of proteins are known to assemble new filaments, including the Arp2/3 complex, which creates branched filament networks, and Spire, which creates unbranched filaments1, 2. We find that JMY, a vertebrate protein first identified as a transcriptional co-activator of p53, combines these two nucleating activities by both activating Arp2/3 and assembling filaments directly using a Spire-like mechanism. Increased levels of JMY expression enhance motility while loss of JMY slows cell migration. When slowly migrating HL-60 cells are differentiated into highly motile neutrophil-like cells, JMY moves from the nucleus to the cytoplasm, and is concentrated at the leading edge. Thus, JMY represents a new class of multifunctional actin assembly factor whose activity is regulated, at least in part, by sequestration in the nucleus. By searching genome databases for sequences related to the WASp Homology 2 (WH2) domain we discovered a potential Arp2/3-activating sequence, WWWCA, in the vertebrate protein JMY (Fig. 1a). This sequence is composed of three tandem repeats of the acti