The role that myosin II plays in altering cell shape and behavior during development in {\it Dictyostelium discoideum\/}

Dictyostelium axenic cell lines have been tested for vegetative motility and cell behavior. The data shows that different cell lines display significant differences in vegetative motility and cell behavior in a substrate-dependent manner. NC4A2 cells behave more like wild type NC4 than do other axenic cell lines. For developmental cells, the interval of cAMP pulsing decreases as development proceeds for wild-type and $mhcA\sp-$ cells. The cAMP pulsing of $mhcA\sp-$ cells is slower than wild type cells. When 2% of fluorescently labeled wild type cells are mixed with $mhcA\sp-$ cells, they have a lower mean rate than when alone, which suggests that the $mhcA\sp-$ cells interfere with wild type cells in cell movement. When 2% of labeled $mhcA\sp-$ are mixed with cells in which one of the two adhesion mechanisms is affected, or double mutants in which both adhesion mechanisms are affected, the $mhcA\sp-$ cells are distorted by interactions with the adhesion mutant cells. $mhcA\sp-$ cells are excluded from streams composed of cells lacking either one of the two adhesion systems but mix randomly with cells lacking both adhesion systems, suggesting that the flaccid cortex of $mhcA\sp-$ cells cannot generate sufficient protrusive force to break the contacts between adhered cells in streams but can enter streams where the cells are weakly adherent. Cell lines lacking either the essential ($mlcE\sp-$) or regulatory ($mlcR\sp-$) myosin light chains have been used to distinguish the two functions of myosin. Neither cells show significant actin activated ATPase activity measured by in vivo or in vitro assays. The $mlcR\sp-$ cells are distorted and localized to the edges of wild type streams indistinguishable from $mhcA\sp-$ cells; the $mlcE\sp-$ cells are randomly mixed in the streams and are not distorted. The similarity of $mlcR\sp-$ and $mhcA\sp-$ cell behavior is consistent with the reduced amount of myosin in the cortex of $mlcR\sp-$ cells. The behavior of the $mlcE\sp-$ cells suggests that actin binding activity and/or normally localized filaments of myosin II in the absence of motor function is sufficient to stiffen the actin cortex in a manner similar to actin crosslinking proteins.