CYTOCHALASIN REARRANGES CORTICAL ACTIN OF THE ALGA NITELLA INTO SHORT, STABLE RODS

The cortical cytoplasm of the alga Nitella contains reticulate actin that does not survive perfusion fixation with glutaraldehyde unless prestabilized with the cross-linker 3-maleimidobenzoyl-N-hydroxysuccinimidester (MBS). Cytochalasin D remodels this cortical actin into short rods which are more stable, surviving aldehyde fixation without MBS pre-treatment. The overall alignment of these actin rods correlates with that of cortical microtubules (transverse in young cells, random in old cells) but probably does not involve one-to-one correspondence. The time course, dose dependence and reversibility of these structural changes broadly resemble those for streaming inhibition by cytochalasin but the cortical actin responds to concentrations that do not slow streaming. Because the structural changes concern the cortical and not the subcortical actin, they seem unlikely to directly inhibit streaming. Formation of cortical rods is not a response to streaming inhibition per se since it does not occur when two other inhibitors of streaming (2,4-dinitrophenol (DNP) and N-ethyl maleimide (NEM)) are used. NEM, however, resembles MBS in stabilizing the reticulate form of cortical actin even though it cannot cross link.