MYOSIN FILAMENT STRUCTURE IN VERTEBRATE SMOOTH-MUSCLE

The in vivo structure of the myosin filaments in vertebrate smooth mus cle is unknown. Evidence from purified smooth muscle myosin and from s ome studies of intact smooth muscle suggests that they may have a nonh elical, side-polar arrangement of crossbridges. However, the bipolar, helical structure characteristic of myosin filaments in striated muscl e has not been disproved for smooth muscle. We have used EM to investi gate this question in a functionally diverse group of smooth muscles ( from the vascular, gastrointestinal, reproductive, and visual systems) from mammalian, amphibian, and avian species, Intact muscle under phy siological conditions, rapidly frozen and then freeze substituted, sho ws many myosin filaments with a square backbone in transverse profile. Transverse sections of fixed, chemically skinned muscles also show sq uare backbones and, in addition, reveal projections (crossbridges) on only two opposite sides of the square. Filaments gently isolated from skinned smooth muscles and observed by negative staining show crossbri dges with a 14.5-nm repeat projecting in opposite directions on opposi te sides of the filament. Such filaments subjected to low ionic streng th conditions show bare filament ends and an antiparallel arrangement of myosin tails along the length of the filament. All of these observa tions are consistent with a side-polar structure and argue against a b ipolar, helical crossbridge arrangement. We conclude that myosin filam ents in all smooth muscles, regardless of function, are likely to be s ide-polar. Such a structure could be an important factor in the abilit y of smooth muscles to contract by large amounts.