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Inorganic phosphate (Pi) and cofilin⁄actin depolymerizing factor proteins have opposite effects on actin filament structure and dynamics. Pi stabilizes the subdomain 2 in F-actin and decreases the critical concentration for actin polymerization. Conversely, cofilin enhances disorder in subdomain 2, increases the critical concentration, and accelerates actin treadmilling. | iFEBS Journal Inorganic phosphate regulates the binding of cofilin to actin filaments Andras Muhlrad1 Dmitry Pavlov2 Y. Michael Peyser1 and Emil Reisler2 1 Institute of DentalSciences Schoolof DentalMedicine Hebrew University of Jerusalem Israel 2 Department of Chemistry and Biochemistry and the Molecular Biology Institute University of California USA Keywords actin cofilin collisionalquenching fluorescence limited proteolysis Correspondence A. Muhlrad Institute of DentalSciences Schoolof DentalMedicine Hebrew University of Jerusalem PO Box 12272 Jerusalem 91120 Israel Fax 972 2 675 8561 Tel 972 2 675 7587 E-mail muhlrad@cc.huji.ac.il Received 3 January 2006 accepted 8 February 2006 doi 10.1111 j.1742-4658.2006.05169.x Inorganic phosphate Pi and coiilin actin depolymerizing factor proteins have opposite effects on actin filament structure and dynamics. Pi stabilizes the subdomain 2 in F-actin and decreases the critical concentration for actin polymerization. Conversely cofilin enhances disorder in subdomain 2 increases the critical concentration and accelerates actin treadmilling. Here we report that Pi inhibits the rate but not the extent of cofilin binding to actin filaments. This inhibition is also significant at physiological concentrations of Pi and more pronounced at low pH. Cofilin prevents conformational changes in F-actin induced by Pi even at high Pi concentrations probably because allosteric changes in the nucleotide cleft decrease the affinity of Pi to F-actin. Cofilin induced allosteric changes in the nucleotide cleft of F-actin are also indicated by an increase in fluorescence emission and a decrease in the accessibility of etheno-ADP to collisional quenchers. These changes transform the nucleotide cleft of F-actin to G-actin-like. Pi regulation of cofilin binding and the cofilin regulation of Pi binding to F-actin can be important aspects of actin based cell motility. Actin dynamics the polymerization and depolymerization of actin filaments and .
