We determined a crystal structure of bovine Arp2/3 complex, an assembly of seven proteins that initiates actin polymerization in eukaryotic cells, at 2.0 angstrom resolution. Actin-related protein 2 (Arp2) and Arp3 are folded like actin, with distinctive surface features. Subunits ARPC2 p34 and ARPC4 p20 in the core of the complex associate through long carboxyl-terminal alpha helices and have similarly folded amino-terminal alpha/beta domains. ARPC1 p40 is a seven-blade beta propeller with an insertion that may associate with the side of an actin filament. ARPC3 p21 and ARPC5 p16 are globular alpha-helical subunits. We predict that WASp/Scar proteins activate Arp2/3 complex by bringing Arp2 into proximity with Arp3 for nucleation of a branch on the side of a preexisting actin filament.

Protein kinase C (PKC) phosphorylates the regulatory light chains of smooth muscle and cytoplasmic myosin II at three known sites: S1, S2, and T9 [Ikebe, M., Hartshorne, D. J., & Elzinga, M. (1987) J. Biol. Chem. 262, 9569-9573]. Phosphorylation at these sites inhibits the actomyosin ATPase and inhibits phosphorylation of S19 on the regulatory light chain by myosin light chain kinase (MLCK) [Nishikawa, M., Sellers, J. R., Adelstein, R. S., & Hidaka, H. (1984) J. Biol. Chem. 259, 8808-8814]. To compare the effects of phosphorylation at a subset of PKC sites on the rate of MLCK phosphorylation, we substituted alanines for the known PKC phosphorylation sites in the Xenopus regulatory light chain (XRLC). PKC phosphorylation of S1A/S2A/T9A revealed secondary phosphorylation sites at T7 and T10, which are accessible both on isolated S1A/S2A/T9A and S1A/S2A/T9A-myosin hybrids. Apparent kinetic constants were determined for MLCK phosphorylation of WT XRLC and XRLC mutants: T9A, S1A/S2A, S1A/S2A/T9A, and T7A/T9A/T10A. PKC prephosphorylation of S1/2 had no effect on the rate of MLCK phosphorylation, while PKC prephosphorylation of T7/9/10 inhibited MLCK phosphorylation due to a 6-fold increase in Km. Our results suggest that phosphorylation of RLC S1/2 as observed in vivo may not be responsible for an inhibition of MLCK phosphorylation.

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