β-Actinin, a Regulatory Protein of Muscle

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Abstract
β-Actinin, a minor regulatory protein of muscle, was purified from skeletal muscles of rabbit and chicken by DEAE-Sephadex chromatography. β-Actinin consisted of two subunits, βI and βII, with chain weights of 37,000 and 34,000 daltons, respectively. The amino acid compositions were similar, though not identical. It appears that each of the two subunits is associated in solution. β-Actinin had the following effects on actin: (1) inhibition of reassociation of F-actin fragments; (2) inhibition of network formation of F-actin; (3) inhibition of growth of F-actin fragments; (4) retardation of depolymerization of F-actin and (5) acceleration of polymerization of G-actin. All these actions of β-actinin can be explained in terms of action as an “endiDg factor.” Experimental evidence favored the view that β-actinin is bound to one end of the F-actin filament, namely to the end opposite to the direction of polymerization. Fluorescence-labeled anti-β-actinin stained the middle portion of the A band of myofibrils. Based on the finding that the statn was unchanged on removal of myosin, it is suggested that β-actinin is located at the free ends of the I filaments of myofibrils. Thus is seems likely that β-actinin functions as an ending factor for actin filaments.

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