Polarity of Filaments

In vitro, it is clear that smooth muscle myosin filaments can assemble into side-polar filaments that are never seen with skeletal muscle myosin (Craig and Megerman, 1977) (Fig. 4). The key feature of a side-polar filament is that all myosin heads have the same polarity along one edge of the filament, and the opposite polarity on the other edge, with bare zones on either end of the filament (Fig. 5). The heads of myosin in a bipolar filament, in contrast, reverse polarity at the filament center, thus producing a central bare zone. 

The intermolecular interactions in a side-polar and bipolar filament differ. Only antiparallel overlaps are needed to generate a side-polar filament. Such interactions in a bipolar filament are limited to the bare zone. In vitro studies clearly show that smooth muscle myosin has a preference for antiparallel interactions. Bipolar smooth muscle myosin filaments greater than 0.5 xm long are generally not formed. Growth beyond this length would require purely parallel interactions between molecules. Short, homogeneous bipolar fila- 

FIGURE 4 Diagram of monomer packing in a side-polar filament. This model suggests that the antiparallel overlap between molecules is 14.3 nm. For clarity, only one of myosin s two heads is shown. Reprinted from Cross et al., Embo. /, 10, 747-756, 1991, by permission of Oxford University Press. 

FIGURE 5 Electron micrographs of negatively stained smooth muscle myosin filaments. Upper panel Smooth muscle myosin rod forms filaments with a distinct side-polar morphology. Lower three panels Myosin filaments formed by addition of 0.1A4 KCl to myosin minifilaments also show a side-polar appearance. Bar-0.1 p,m. Reproduced from the Journal of Cell Biology, 1987, 105, 3007-3019 by copyright permission of The Rockefeller University Press. 

The length of the antiparallel overlap between molecules in a side-polar filament is not firmly established. A 43-nm antiparallel overlap was unique to segments formed from smooth muscle myosin rods by precipitation with divalent cations (Kendrick-Jones et al., 1971). Folded dimers with an approximately 40- to 50-nm antiparallel overlap are formed at salt concentrations 50 mM KCl (Trybus and Lowey, 1984). Both of these observations suggest that the molecule favors a 43-nm overlap. Measurements obtained by scanning transmission electron microscopy, however, show that filaments formed in vitro have one antiparallel dimer per 14.3 nm of filament length, a value that favors a model where adjacent molecules have a 14.3-nm antiparallel overlap (Cross and Engel, 1991) (Fig. 4). The 

---