Dynamics of Actin and α-Actinin in Nascent Myofibrils and Stress Fibers

Actin labeled with fluorescein isothiocyanate (FITC) and α-actinin labeled with rhodamine (rh) were co-injected into chick embryonic cardiac myocytes and fibroblasts. In cardiomyocytes, FITC-actin was distributed in nonstriated lines, linearly arranged punctate structures with short intervals, and cross-striated bands with regular sarcomeric intervals. rh-α-Actinin was seen to be distributed in the same pattern in the former two portions, and in the center of each striation in the latter portion. Photobleaching of structures incorporated with these fluorescent analogs revealed that the fluorescent recovery rate of actin decreased in the order of nonstriated > punctated > striated portions, while that of α-actinin was low and stable at all portions. During the transition phase from punctate to regular sarcomere structures of these proteins, short spaced α-actinin dots adjoined each other and aligned with Z bands of neighboring myofibrils. It appears that both the difference in exchangeability between actin and α-actinin molecules and the movement of α-actinin dots during this phase of myofibrillogenesis are related to sarcomere lengthening and I–Z–I brush formation; adjoining dots of low-exchangeable α-actinin may provide favorable situations for exchangeable actin molecules in filaments to elongate and/or rearrange. In fibroblasts, both FITC-actin and rh-α-actinin formed nonstriated lines. In these cells, exchangeabilities of both proteins were high and similar in rate. This seems to indicate that stress fibers are constantly exchanging their components for motile and other vital functions of these cells. The high exchangeabilities of both proteins in stress fibers show that these fibers are clearly different from nonstriated, stress-fiber like structures of nascent myofibrils.