MAMMALIAN ACTIN-RELATED PROTEIN 2 3 COMPLEX LOCALIZES TO REGIONS OF LAMELLIPODIAL PROTRUSION AND IS COMPOSED OF EVOLUTIONARILY CONSERVED PROTEINS/

Human neutrophils contain a complex of proteins similar to the actin-r elated protein 2/3 (Arp2/3) complex of Acanthamoeba. We have obtained peptide sequence information for each member of the putative seven-pro tein complex previously described for Acanthamoeba and human platelets . From the peptide sequences we have identified cDNA Species encoding three novel proteins in this complex. We find that in addition-to Arp2 and Arp3, this complex contains a relative of the human (Suppressor o f Profilin) SOP2Hs protein and four previously unknown proteins. These proteins localize in the cytoplasm of fibroblasts that lack lamellipo dia, but are enriched in lamellipodia on stimulation with serum or pla telet-derived growth factor. We propose a conserved and dynamic role f or this complex in the organization of the actin cytoskeleton.