MECHANICAL PERTURBATION ELICITS A PHENOTYPIC DIFFERENCE BETWEEN DICTYOSTELIUM WILD-TYPE CELLS AND CYTOSKELETAL MUTANTS

To determine the specific contribution of cytoskeletal proteins to cel lular viscoelasticity we performed rheological experiments with Dictyo stelium discoideum wild-type cells (AX2) and mutant cells altered by h omologous recombination to lack alpha-actinin (AHR), the ABP120 gelati on factor (GHR), or both of these F-actin cross-linking proteins (AGHR ). Oscillatory and steady flow measurements of Dictyostelium wild-type cells in a torsion pendulum showed that there is a large elastic comp onent to the viscoelasticity of the cell pellet, Quantitative rheologi cal measurements were performed with an electronic plate-and-cone rheo meter, which allowed determination of G', the storage shear modulus, a nd G'', the viscous loss modulus, as a function of time, frequency, an d strain, respectively. Whole cell viscoelasticity depends strongly on all three parameters, and comparison of wild-type and mutant strains under identical conditions generally produced significant differences, Especially stress relaxation experiments consistently revealed a clea r difference between cells that lacked oc-actinin as compared with wil d-type cells or transformants without ABP120 gelation factor, indicati ng that cu-actinin plays an important role in cell elasticity. Direct observation of cells undergoing shear deformation was done by incorpor ating a small number of AX2 cells expressing the green fluorescent pro tein of Aequorea victoria and visualizing the strained cell pellet by fluorescence and phase contrast microscopy. These observations confirm ed that the shear strain imposed by the rheometer does not injure the cells and that the viscoelastic response of the cell pellet is due to deformation of individual cells.