A rotation model for microtubule and filament sliding.

Simple model experiments show that the cyclic motion of myosin cross-bridges in muscle which is assumed to be active ("sliding model" by "power-stroke" or "rowing-stroke" of the crossbridges) can be interpreted equally well as a passive process during which the myosin heads simply lock mechanically into the grooves of the thin filaments. In order to explain the sliding process a filament or microtubule rotation is assumed to be combined with the winding and unwinding of associated helical protein filaments ("MAPs", "dynein"). As shown in further model experiments the direction of helix winding or unwinding along a rod (microtubule) determines the direction of rod displacement ("parallel" or "antiparallel sliding"). The "sidearms" and "bridges" visible in the electron microscope along the cytoskeletal elements might correspond to the winding or unwinding filaments. On the basis of this conception simple models for the behavior of spindle microtubules and the anaphase movement of chromosomes are presented. The latter is assumed to occur via the unwinding of helical filaments accompanying the kinetochore microtubules, which causes their simultaneous depolymerization.