ACTIN-MICROTUBULE INTERACTIONS IN THE ALGA NITELLA - ANALYSIS OF THE MECHANISM BY WHICH MICROTUBULE DEPOLYMERIZATION POTENTIATES CYTOCHALASINS EFFECTS ON STREAMING

In the characean alga Nitella, depolymerization of microtubules potent iates the inhibitory effects of cytochalasins on cytoplasmic streaming . Microtubule depolymerization lowers the cytochalasin B and D concent rations required to inhibit streaming, accelerates inhibition and dela ys streaming recovery. Because microtubule depolymerization does nor s ignificantly alter H-3-cytochalasin B uptake and release, elevated int racellular cytochalasin concentrations are not the basis for potentiat ion. Instead, microtubule depolymerization causes actin to become more sensitive to cytochalasin. This increased sensitivity of actin is unl ikely to be due to direct stabilization of actin by microtubules, howe ver, because very few microtubules colocalize with the subcortical act in bundles that generate streaming. Furthermore, microtubule reassembl y, but nor recovery of former transverse alignment, is sufficient for restoring the normal cellular responses to cytochalasin D. We hypothes ize that either tubulin or microtubule-associated proteins, released w hen microtubules depolymerize, interact with the actin cytoskeleton an d sensitize it to cytochalasin.