Analysis of myofibrillar structure and assembly using fluorescently labeled contractile proteins.

Abstract

To study how contractile proteins become organized into sarcomeric units in striated [rabbit] muscle, glycerinated myofibrils were exposed to fluorescently labeled actin, .alpha.-actinin and tropomyosin. In this in vitro system, .alpha.-actinin bound to the Z-bands and the binding could not be saturated by prior addition of excess unlabeled .alpha.-actinin. Conditions known to prevent self-association of .alpha.-actinin, however, blocked the binding of fluorescently labeled .alpha.-actinin to Z-bands. When tropomyosin was removed from the myofibrils, .alpha.-actinin then added to the thin filaments as well as the Z-bands. Actin bound in a doublet pattern to the regions of the myosin filaments where there were free cross-bridges i.e., in that part of the A-band free of interdigitating native thin filaments but not in the center of the A-band which lacks cross-bridges. In the presence of 0.1-0.2 mM ATP, no actin binding occurred. When unlabeled .alpha.-actinin was added first to myofibrils and then labeled actin was added, flourescence occurred not in a doublet pattern but along the entire length of the myofibril. Tropomyosin did not bind to myofibrils unless the existing tropomyosin was 1st removed, in which case it added to the thin filaments in the I-band. Tropomyosin did bind, however, to the exogenously added tropomyosin-free actin that localizes as a doublet in the A-band. Evidently, the .alpha.-actinin present in Z-bands of myofibrils is fully complexed with actin, but can bind exogenous .alpha.-actinin and, if actin is added subsequently, the exogenous .alpha.-actinin in the Z-band will bind the newly formed fluorescent actin filaments. Myofibrillar actin filaments did not increase in length when G-actin was present under polymerizing conditions, nor did they bind any added tropomyosin. These observations are discussed in terms of the structure and in vivo assembly of myofibrils.