title: Coordinated regulation of platelet actin filament barbed ends by gelsolin and capping protein.
abstract: Exposure of cryptic actin filament fast growing ends (barbed ends) initiates actin polymerization in stimulated human and mouse platelets. Gelsolin amplifies platelet actin assembly by severing F-actin and increasing the number of barbed ends. Actin filaments in stimulated platelets from transgenic gelsolin-null mice elongate their actin without severing. F-actin barbed end capping activity persists in human platelet extracts, depleted of gelsolin, and the heterodimeric capping protein (CP) accounts for this residual activity. 35% of the approximately 5 microM CP is associated with the insoluble actin cytoskeleton of the resting platelet. Since resting platelets have an F-actin barbed end concentration of approximately 0.5 microM, sufficient CP is bound to cap these ends. CP is released from OG-permeabilized platelets by treatment with phosphatidylinositol 4,5-bisphosphate or through activation of the thrombin receptor. However, the fraction of CP bound to the actin cytoskeleton of thrombin-stimulated mouse and human platelets increases rapidly to approximately 60% within 30 s. In resting platelets from transgenic mice lacking gelsolin, which have 33% more F-actin than gelsolin-positive cells, there is a corresponding increase in the amount of CP associated with the resting cytoskeleton but no change with stimulation. These findings demonstrate an interaction between the two major F-actin barbed end capping proteins of the platelet: gelsolin-dependent severing produces barbed ends that are capped by CP. Phosphatidylinositol 4,5-bisphosphate release of gelsolin and CP from platelet cytoskeleton provides a mechanism for mediating barbed end exposure. After actin assembly, CP reassociates with the new actin cytoskeleton.
authors.affiliation: Division of Experimental Medicine, Brigham and Women's Hospital, Boston, Massachusetts 02115, USA.
authors.forename: K
authors.initials: K
authors.lastname: Barkalow
authors.forename: W
authors.initials: W
authors.lastname: Witke
authors.forename: D J
authors.initials: DJ
authors.lastname: Kwiatkowski
authors.forename: J H
authors.initials: JH
authors.lastname: Hartwig
mesh_terms: D008841:Actin Cytoskeleton / Q000378:metabolism; D051339:Actin Depolymerizing Factors; D000199:Actins / Q000378:metabolism*; D000818:Animals; D001792:Blood Platelets / Q000187:drug effects / Q000378:metabolism* / Q000648:ultrastructure; D051347:CapZ Actin Capping Protein; D002463:Cell Membrane Permeability; D002645:Chickens; D003285:Contractile Proteins / Q000378:metabolism; D003599:Cytoskeleton / Q000378:metabolism; D051345:Destrin; D064448:Filamins; D018260:Gelsolin / Q000378:metabolism*; D005960:Glucosides / Q000494:pharmacology; D006801:Humans; D006863:Hydrogen-Ion Concentration; D051379:Mice; D008840:Microfilament Proteins / Q000378:metabolism*; D009124:Muscle Proteins; D019269:Phosphatidylinositol 4,5-Diphosphate; D018129:Phosphatidylinositol Phosphates / Q000494:pharmacology; D015539:Platelet Activation; D018179:Receptors, Thrombin / Q000378:metabolism
chemical_list: D051339:Actin Depolymerizing Factors; D000199:Actins; C495842:CAPZA1 protein, human; D051347:CapZ Actin Capping Protein; C495843:Capza1 protein, mouse; C495847:Capza2 protein, mouse; D003285:Contractile Proteins; D051345:Destrin; D064448:Filamins; D018260:Gelsolin; D005960:Glucosides; D008840:Microfilament Proteins; D009124:Muscle Proteins; D019269:Phosphatidylinositol 4,5-Diphosphate; D018129:Phosphatidylinositol Phosphates; D018179:Receptors, Thrombin; C018619:octyl-beta-D-glucoside