DICTYOSTELIUM CELL-SHAPE GENERATION REQUIRES MYOSIN-II

Myosin II function has been implicated in non-muscle cell behaviors in cluding movement, cytokinesis, and multicellular morphogenesis. Surpri singly, two-dimensional morphology and behavior of Dictyostelium amoeb ae lacking myosin LI (mhcA(-)) is not dramatically altered when cells are examined using conventional imaging techniques. We have observed a moebae from the side (side-view microscopy or SVM) and find that wild- type but not mhcA(-) cells undergo extensive three-dimensional (3D) sh ape changes. These shape changes often occur above the plane of the su bstrate and are morphologically distinct from pseudopods. For example, unlike pseudopods, vertically extended cell regions are not enriched in F-actin and do not exclude organelles. In contrast, mhcA(-) cells g enerate F-actin-filled pseudopods and spread laterally but do not unde rgo vertical extension. When wild-type cells are removed from the subs trate and shaken in suspension, they retain their irregular three-dime nsional shape. Suspended mhcA(-) cells, however, rapidly become spheri cal. Thus, cells lacking myosin appear to be incapable of generating o r maintaining 3D shape independent of a substrate. When a substrate is available, these cells can attach and spread on the surface in a myos in-independent manner. These previously undescribed defects of mhcA(-) cells reveal that 3D cell shape generation requires myosin II and is mechanistically distinct from pseudopod formation. (C) 1996 Wiley-Liss , Inc.