DYNAMICS OF ACTIN AND ASSEMBLY OF CONNECTIN (TITIN) DURING MYOFIBRILLOGENESIS IN EMBRYONIC CHICK CARDIAC-MUSCLE-CELLS IN-VITRO

Immunogold electron microscopy of cardiac myocytes microinjected with biotin-labeled actin showed that gold labeling was first found around the A band level of myofibrils at their proximal parts. This observation suggests that polymerization of actin and/or the addition of newly formed actin filaments occurs preferentially in association with myosin filaments to increase the myofibrillar girth. At the distal portions of developing myofibrils, their terminal ends were initially labeled, suggesting that continued reorganization and/or de novo formation of myofibrils occurs at these locations. Soon, gold particles were seen along the termini of growing myofibrils. This appears to indicate that actin subunits are added at the membrane-associated ends of preexisting actin filaments to increase the length of myofibrils. Adhesion plaque proteins, e.g., vinculin, do not appear to play any role in assembling actin monomers at these sites on the inner surface of the sarcolemma. Immunofluorescence and immunoelectron microscopy of cardiomyocytes double-stained with antibodies against two distant domains of connectin (titin) filaments and other sarcomeric proteins showed that these domains of connectin filaments and myosin were synthesized almost simultaneously on large polyribosomes and/or associated immediately after the synthesis of these molecules. Connectin and myosin bands were formed after alpha-actinin striations (Z bands) were seen on preformed I-Z-I-like structures. The observation that the development and distribution of connectin were tightly linked with those of myosin suggests the possible role of connectin for integrating myosin filaments with the early formed I-Z-I complexes of myofibrils. (C) 1993 Wiley-Liss, Inc.