Structural basis of contraction in vertebrate smooth muscle

Abstract

The present report confirms and extends the earlier evidence (Lowy & Small 1970) for ribbon-shaped elements being the in vivo form of the myosin component of vertebrate smooth muscle. Thus, it is shown (1) that the ribbons have a highly ordered substructure and (2) that two other filament types also found in the cells and described by previous investigators are most readily explained as disaggregation or breakdown products of the ribbons, formed as a result of chemical fixation. The results are taken mainly from the taenia coli muscle but also from the vas deferens of the guinea pig. The ribbons are shown structurally to be composed of two components; (1) a filamentous, ribbon-shaped backbone and (2) a surface lattice of myosin molecules. The backbone is made up from a side-by-side aggregation of small filaments that are also found free in the cytoplasm and which are square in cross-section, about 75 Å across and composed of four filamentous sub-units, each of about 35 Å diameter. These filaments, which we term “lentofilaments”, resemble in size and ultrastructure the neurofilaments and tonofilaments in other cell types and which, in certain instances are thought to be composed of keratin. In longitudinal sections the ribbons are seen to possess a very regular arrangement of projections, which are presumed to represent the heads of myosin molecules. Section tilting shows these to be organized in rows across each ribbon face with the rows spaced axially at a constant period of about 140 Å. Optical diffraction analysis indicates that the lattice on the ribbon seen in face view has a unit cell with approximate dimensions, a = 100 A ̊ (across the face), b = 150 A ̊ , σ = 74 °, and contains one projection. This corresponds to an axial repeat of about 720 Å and a side-separation of 100 Å between projections. Longitudinal sections also show the ribbons to be extremely long, and probably greater than 5 μm in length. The dense areas, or dense bodies, are not attachment sites for the thin filaments but are considered to be artifacts of preparation; it is suggested they consist of denatured myosin. It is shown that the ribbons do not result from the exposure of muscles to hypertonic bathing media, or from treatment at low temperature or from the use of excessive loads, as has been suggested. From optical diffraction analysis of groups of thin filaments in transverse section, it is demonstrated that within each group the thin filaments are arranged in a fairly regular hexagonal lattice. However, there is little or no correlation between the order in one group and that in other groups in the same cell. These conclusions are compatible with published X-ray diffraction evidence. Optical diffraction analysis of ribbons seen in “edge view” in longitudinal section indicates that the bridges that project from opposite faces of the ribbon are directed towards opposite ends of the ribbon, that is, there is a reversed polarity of bridges on one face with respect to the other. A sliding filament model for the contractile mechanism of smooth muscle is thus proposed, whereby actin filaments that interact with opposite faces of a ribbon slide in opposite directions.