2 DISTINCT ISOFORMS OF SEA-URCHIN EGG DYNEIN

Extracts of unfertilized sea urchin eggs contain at least two isoforms of cytoplasmic dynein. One exhibits a weak affinity for microtubules and is primarily soluble. The other isoform, HMr-3, binds to microtubu les in an ATP-sensitive manner, but is immunologically distinct from t he soluble egg dynein (Porter et al.: Journal of Biological Chemistry 263:6759-6771, 1988). We have now further distinguished these egg dyne in isoforms based on differences in NTPase activity. HMr-3 copurifies with NTPase activity, but it hydrolyzes CTP at 10 times the rate of AT P. The soluble egg dynein is similar to flagellar dynein in its nucleo tide specificity; its MgCTPase activity is ca. 60% of its MgATPase act ivity. Non-ionic detergents and salt activate the MgATPase activities of both enzymes relative to their MgCTPase activities, but this effect is more pronounced for the soluble egg dynein than for HMr-3. Sucrose gradient-purified HMr-3 promotes an ATP-sensitive microtubule bundlin g, as seen with darkfield optics. We have also isolated a 20 S microtu bule translocating activity by sucrose gradient fractionation of egg e xtracts, followed by microtubule affinity and ATP release. This 20 S f raction, which contains the HMr-3 isoform, induces a microtubule glidi ng activity that is distinct from kinesin. Our observations suggest th at soluble dynein resembles axonemal dynein, but that HMr-3 is related to the dynein-like enzymes isolated from a variety of cell types and may represent the cytoplasmic dynein of sea urchin eggs.