Activation of dynamin GTPase is a result of positive cooperativity

Dynamin is a GTP-binding protein thought to be involved in the early stages of endocytosis. Presently, it is not known how dynamin GTP binding and hydrolysis are related to its role in this process. We previously characterized the ability of acidic phospholipid vesicles and microtubules to strongly stimulate the GTPase activity of purified brain dynamin. In a further analysis of dynamin enzymatic properties, we have found that the increase of dynamin GTP hydrolysis in the presence of activating agent depends on enzyme concentration. At low enzyme concentration, little or no activation is observed. Plots of dynamin GTPase activity with increasing enzyme concentration in the presence of either activating agent are strongly sigmoidal, indicating that positive cooperativity 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 effects in GTP binding were observed. Linear transformation of reaction velocity versus enzyme concentration data indicate an apparent Km for dynamin-dynamin interactions of 37 nM, which is significantly lower than the physiological concentration of dynamin in brain. These results suggest that cooperative interactions between dynamin molecules are responsible for the apparent activation of GTPase observed and are likely involved in dynamin function in vivo.