ACTIVATION OF DYNAMIN GTPASE IS A RESULT OF POSITIVE COOPERATIVITY

Dynamin is a GTP-binding protein thought to be involved in the early s tages of endocytosis. Presently, it is not known how dynamin GTP bindi ng and hydrolysis are related to its role in this process. We previous ly characterized the ability of acidic phospholipid vesicles and micro tubules to strongly stimulate the GTPase activity of purified brain dy namin. In a further analysis of dynamin enzymatic properties, we have found that the increase of dynamin GTP hydrolysis in the presence of a ctivating agent depends on enzyme concentration. At low enzyme concent ration, Little or no activation is observed. Plots of dynamin GTPase a ctivity with increasing enzyme concentration in the presence of either activating agent are strongly sigmoidal, indicating that positive coo perativity is responsible for the increased activity observed. A Hill coefficient of 2.3 was determined, implying that at least two dynamin molecules associate for maximal GTPase activity. No cooperative effect s in GTP binding were observed. Linear transformation of reaction velo city versus enzyme concentration data indicate an apparent II, for dyn amin-dynamin interactions of 37 nM, which is significantly lower than the physiological concentration of dynamin in brain. These results sug gest that cooperative interactions between dynamin molecules are respo nsible for the apparent activation of GTPase observed and are likely i nvolved in dynamin function in vivo.