EVIDENCE FOR OPPOSING EFFECTS OF CALMODULIN ON CORTICAL MICROTUBULES

Microtubule integrity within the cortical array was visualized in dete rgent-lysed carrot (Daucus carota L.) protoplasts that were exposed to various exogenous levels of Ca2+ and calmodulin (CaM). CaM appears to help stabilize cortical microtubules against the destabilizing action of Ca2+/CaM complexes at low Ca2+ concentrations, but not at higher C a2+ concentrations. The hypothesis that CaM interacts with microtubule s at two different sites, determined by the concentration of Ca2+, is supported by the effects of the CaM antagonists N-(6-aminohexyl)-1-nap hthalene-sulfonamide and N-(6-aminohexyl)-5-chloro-1-naphthalenesulfan amide (20 mu M) and by affinity chromatography. Two classes of protein s were identified that interact with tubulin and bind to CaM. One clas s required Ca2+ for CaM binding, whereas the second class bound only w hen Ca2+ concentrations were low (<320 nM). Thus, CaM's ability to hav e two opposing effects upon microtubules may be regulated by the conce ntration of intracellular Ca2+ and its differential interactions with microtubule-associated proteins. Experimental manipulation of intracel lular Ca2+ concentrations, as monitored by Indo-1, revealed that the e ffect of Ca2+ is specific to the cortical microtubules and does not af fect actin microfilaments in these cells.