Phosphorylation of the Arp2 is regulated in cells. (A) Immunoprecipitation of the Arp2/3 complex with antibodies to ARPC1 from MTLn3 epithelial cells metabolically labeled with [³²P]orthophosphate indicated that the Arp2 subunit is phosphorylated in unstimulated cells, and phosphorylation increased after stimulation with EGF. (B) Quantification of time-dependent Arp2 phosphorylation. Data are expressed as the fold increase of phosphorylation in the absence of EGF and represent means ± SD of five separate cell preparations. (C) The Arp2/3 complex was immunoprecipitated from quiescent and EGF-stimulated MTLn3 cells. Labeling of Arp2 with antibodies to pThr and pTyr increased ∼1.3- and 1.21-fold after EGF stimulation, respectively.