title: Interaction of thymosin beta 4 with muscle and platelet actin: implications for actin sequestration in resting platelets.
abstract: Quantitative measurements of the interactions of T beta 4 with muscle actin suggest that its only physiological role is monomer sequestration. T beta 4 forms a 1:1 complex with monomeric actin under physiological salt conditions. Its Kd for actin is not affected by calcium. T beta 4 binds only to actin monomers and not to filament ends or alongside the filament. T beta 4-actin complexes do not elongate actin filaments at either the barbed or the pointed end, and, unlike actobindin, T beta 4 does not specifically suppress the nucleation of polymerization. We assessed the fraction of monomeric actin that can be sequestered by T beta 4 in resting platelets. This was done on the basis of (a) its Kd of 0.4-0.7 microM for platelet actin, which had been prepared by a newly devised simpler method, and (b) the values for the concentrations of monomeric actin and of T beta 4 which we measured as 280 and 560 microM, respectively. Using the higher Kd value of 0.7 microM, the T beta 4-complexed actin is calculated to be between 70 and 240 microM, depending on the steady-state free G-actin concentration. This may vary from 0.1 to 0.5 microM, the critical concentrations for uncapped and for fully barbed-end-capped actin filaments. If the Kd in the platelet is the same as in vitro, most of the sequestered actin would be bound to T beta 4 if more than 95% of the actin filaments are capped at their barbed ends in resting platelets.
authors.affiliation: Department of Biochemistry and Biophysics, University of Pennsylvania, Philadelphia 19104.
authors.forename: A
authors.initials: A
authors.lastname: Weber
authors.forename: V T
authors.initials: VT
authors.lastname: Nachmias
authors.forename: C R
authors.initials: CR
authors.lastname: Pennise
authors.forename: M
authors.initials: M
authors.lastname: Pring
authors.forename: D
authors.initials: D
authors.lastname: Safer
mesh_terms: D000199:Actins / Q000097:blood / Q000302:isolation & purification / Q000378:metabolism*; D000818:Animals; D001792:Blood Platelets / Q000378:metabolism*; D002850:Chromatography, Gel; D006801:Humans; D007700:Kinetics; D046911:Macromolecular Substances; D009132:Muscles / Q000378:metabolism*; D011817:Rabbits; D013947:Thymosin / Q000031:analogs & derivatives* / Q000378:metabolism / Q000494:pharmacology; D000099201:Thymosin beta(4)
chemical_list: D000199:Actins; D046911:Macromolecular Substances; D013947:Thymosin; D000099201:Thymosin beta(4)