Aggregation of thick filaments into ribbons in mammalian smooth muscle.

Abstract

THE thick filaments in electron micrographs of smooth muscle account quantitatively for its biochemically demonstrable myosin content1–3 and optical diffraction combined with electron microscopy has revealed a regular lateral spacing at approximately 700 A of thick filaments in rabbit portal mesenteric vein (MV) and guinea-pig taenia coli (TC) (see previous article by Rice et al.4). The belief that these thick filaments represent the organized form of myosin in mammalian smooth muscle is supported by the X-ray diffraction data which indicate a 144 A meridional reflexion in living TC5, but there is an important discrepancy between the descriptions of the thick filaments characterized in our laboratories and the ribbon-like structures described by Lowy and Small6 as the normal form of myosin in smooth muscle. We have therefore investigated the mechanisms that could lead to the aggregation of what we consider to be normal regular arrays of thick filaments into disordered ribbon-like structures.