Mechanism of myosin subfragment-1-induced assembly of CaG-actin and MgG-actin into F-actin-S1-decorated filaments.
The kinetics and mechanism of myosin subfragment-1-induced polymerization of G-actin into F-actin-S1-decorated filaments have been investigated in low ionic strength buffer and in the absence of free ATP. The mechanism of assembly of F-actin-S1 differs from salt-induced assembly of F-actin. Initial condensation of G-actin and S1 into oligomers in reversible equilibrium is a prerequisite step in the formation of F-actin-S1 . Oligomers have a relatively low stability (10(6) M-1) and contain S1 in a molar ratio to actin close to 0.5. Increased binding of S1 up to a 1:1 molar ratio to actin is associated with further irreversible condensation of oligomers into large F-actin-S1 structures of very high stability. In contrast to salt-induced assembly of F-actin, no monomer-polymer equilibrium, characterized by a critical concentration, can be defined for F-actin-S1 assembly, and end-to-end annealing of oligomers is predominant over growth from nuclei in the kinetics. Simultaneous recordings of the changes in light scattering, pyrenyl- and NBD-actin fluorescence, ATP hydrolysis, and release of Pi during the polymerization process have been analyzed to propose a minimum kinetic scheme for assembly, within which several elementary steps, following oligomer formation, are required for assembly of F-actin-S1. ATP hydrolysis occurs before polymerization of MgATP-G-actin but not of CaATP-G-actin. The release of inorganic phosphate occurs on F-actin-S1 at the same rate as on F-actin.
Fievez
S
S
Laboratoire d'Enzymologie et Biochimie Structurales, CNRS, 91198 Gif-sur-Yvette, France.
Carlier
M F
MF
Pantaloni
D
D
D008841:Actin Cytoskeleton / Q000378:metabolism*; D000199:Actins / Q000378:metabolism*; D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D007700:Kinetics; D008027:Light; D008274:Magnesium / Q000378:metabolism*; D008956:Models, Chemical; D015879:Myosin Subfragments / Q000378:metabolism*; D011108:Polymers / Q000378:metabolism; D011817:Rabbits; D012542:Scattering, Radiation; D013050:Spectrometry, Fluorescence
D000199:Actins; D015879:Myosin Subfragments; D011108:Polymers; D000255:Adenosine Triphosphate; D008274:Magnesium; D002118:Calcium