Nonmuscle myosin II localizes to the Z-lines and intercalated discs of cardiac muscle and to the Z-lines of skeletal muscle

To understand the role of nonmuscle myosin II in cardiac and skeletal muscl e, we used a number of polyclonal antibodies, three detecting nonmuscle myo sin heavy chain II-B (NMHC II-B) and two detecting NMHC II-A, to examine th e localization of these two proteins in fresh-frozen, acetone-fixed section s of normal human and mouse hearts and human skeletal muscles. Results were similar in both species and were confirmed by examination of fresh-frozen sections of human hearts subjected to no fixation or to treatment with eith er 4% p-formaldehyde or 50% glycerol. NMHC II-B was diffusely distributed i n the cytoplasm of cardiac myocytes during development, but after birth it was localized to the Z-lines and intercalated discs. Dual labeling showed a lmost complete colocalization of NMHC II-B with alpha-actinin. Whereas endo thelial cells, smooth muscle cells and fibroblasts showed strong immunoreac tivity for NMHC II-A and NMHC II-B, cardiac myocytes only showed reactivity for the latter. The Z-lines of human skeletal muscle cells, in contrast to those of cardiac myocytes, gave positive reactions for both NMHC II-A and NMHC II-B. The presence of a motor protein in the Z-lines and intercalated discs raises the possibility that these structures may play a more dynamic role in the contraction/relaxation mechanism of cardiac and skeletal muscle than has been previously suspected. (C) 2000 Wiley-Liss, Inc.