ABSTRACT
Expression of human profilin-I does not complement the temperature-sensitive cdc3-124 mutation of the single profilin gene in fission yeast Schizosaccharomyces pombe, resulting in death from cytokinesis defects. Human profilin-I and S. pombe profilin have similar affinities for actin monomers, the FH1 domain of fission yeast formin Cdc12p and poly-L-proline (Lu, J., and Pollard, T. D. (2001) Mol. Biol. Cell 12, 1161-1175), but human profilin-I does not stimulate actin filament elongation by formin Cdc12p like S. pombe profilin. Two crystal structures of S. pombe profilin and homology models of S. pombe profilin bound to actin show how the two profilins bind to identical surfaces on animal and yeast actins even though 75% of the residues on the profilin side of the interaction differ in the two profilins. Overexpression of human profilin-I in fission yeast expressing native profilin also causes cytokinesis defects incompatible with viability. Human profilin-I with the R88E mutation has no detectable affinity for actin and does not have this dominant overexpression phenotype. The Y6D mutation reduces the affinity of human profilin-I for poly-l-proline by 1000-fold, but overexpression of Y6D profilin in fission yeast is lethal. The most likely hypotheses to explain the incompatibility of human profilin-I with Cdc12p are differences in interactions with the proline-rich sequences in the FH1 domain of Cdc12p and wider "wings" that interact with actin.
Subject(s)
Cytoskeletal Proteins/chemistry , Cytoskeletal Proteins/metabolism , Profilins/chemistry , Profilins/metabolism , Schizosaccharomyces pombe Proteins/chemistry , Schizosaccharomyces pombe Proteins/metabolism , Schizosaccharomyces/chemistry , Schizosaccharomyces/metabolism , Actins/chemistry , Actins/metabolism , Amino Acid Sequence , Animals , Binding Sites , Cattle , Cell Cycle Proteins/genetics , Cell Cycle Proteins/metabolism , Crystallography, X-Ray , Cytoskeletal Proteins/genetics , Humans , Ligands , Models, Molecular , Molecular Sequence Data , Mutation/genetics , Phenotype , Phosphatidylinositol Phosphates/metabolism , Profilins/genetics , Protein Structure, Tertiary , Schizosaccharomyces/genetics , Schizosaccharomyces pombe Proteins/genetics , Sequence Alignment , Structural Homology, Protein , TemperatureABSTRACT
Members of the Wiskott-Aldrich syndrome protein (WASP) family link Rho GTPase signaling pathways to the cytoskeleton through a multiprotein assembly called Arp2/3 complex. The C-terminal VCA regions (verprolin-homology, central hydrophobic, and acidic regions) of WASP and its relatives stimulate Arp2/3 complex to nucleate actin filament branches. Here we show by differential line broadening in NMR spectra that the C (central) and A (acidic) segments of VCA domains from WASP, N-WASP and Scar bind Arp2/3 complex. The C regions of these proteins have a conserved sequence motif consisting of hydrophobic residues and an arginine residue. Point mutations in this conserved sequence motif suggest that it forms an amphipathic helix that is required in biochemical assays for activation of Arp2/3 complex. Key residues in this motif are buried through contacts with the GTPase binding domain in the autoinhibited structure of WASP and N-WASP, indicating that sequestration of these residues is an important aspect of autoinhibition.
