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PubMed1998

SCAR, a WASP-related protein, isolated as a suppressor of receptor defects in late Dictyostelium development.

G protein-coupled receptors trigger the reorganization of the actin cytoskeleton in many cell types, but the steps in this signal transduction cascade are poorly understood. During Dictyostelium development, extracellular cAMP functions as a chemoattractant and morphogenetic signal that is transduced via a family of G protein-coupled receptors, the cARs. In a strain where the cAR2 receptor gene is disrupted by homologous recombination, the developmental program arrests before tip formation. In a genetic screen for suppressors of this phenotype, a gene encoding a protein related to the Wiskott-Aldrich Syndrome protein was discovered. Loss of this protein, which we call SCAR (suppressor of cAR), restores tip formation and most later development to cAR2(-) strains, and causes a multiple-tip phenotype in a cAR2(+) strain as well as leading to the production of extremely small cells in suspension culture. SCAR-cells have reduced levels of F-actin staining during vegetative growth, and abnormal cell morphology and actin distribution during chemotaxis. Uncharacterized homologues of SCAR have also been identified in humans, mouse, Caenorhabditis elegans, and Drosophila. These data suggest that SCAR may be a conserved negative regulator of G protein-coupled signaling, and that it plays an important role in regulating the actin cytoskeleton.

Amino Acid Sequence↗
PubMed2004

Apical localization of actin patches and vacuolar dynamics in Ashbya gossypii depend on the WASP homolog Wal1p.

Analysis of the Ashbya gossypii Wiskott-Aldrich syndrome-like gene AgWAL1 indicates that it is required for the maintenance of polarized hyphal growth. Growth and organelle dynamics of the wild type and of wal1 and other mutant strains were monitored by in vivo (fluorescence) time-lapse microscopy. Loss of WAL1 led to slow growth and defects in polarized growth that produced swellings in subapical regions, whereas formation of hyphal tips and dichotomous tip branching occurred as in the wild-type. Few actin cables in Agwal1 cells were found to insert into the hyphal tip, but specific clustering of cortical actin patches was observed in subapical regions of hyphal tips instead of at the hyphal apex. Distribution and movement of vacuoles was observed in vivo using FM4-64. In the wild type and in the slowly growing mutant strains bem2 and cla4, which lack a Rho-GTPase-activating protein and a PAK kinase, respectively, early endosomes appeared in the hyphal tip, whereas very few early endosomes and small vacuoles were found in the wal1 mutant hyphal tips, thus linking the cortical patch defect of wal1 hyphae with the distribution of endosomes. Vivid movement of vacuoles seen in the wild type and in the bem2 mutant in subapical regions was largely reduced in the wal1 and cla4 mutants. The tubular structure of mitochondria (as visualized by DIOC6 in vivo) was similar in the wild type and the wal1 mutant, although wal1 mitochondria appeared to be larger. Interestingly, mitochondria were found to insert into the hyphal tips in both strains. Our results indicate a function for Wal1p in filamentous fungi in coordinating actin patch distribution with polarized hyphal tip growth.

Actins↗