Dissecting dynamin's role in clathrin-mediated endocytosis

Abstract

The GTPase dynamin is essential for CME (clathrin-mediated endocytosis), but its exact function and mechanism of action have been controversial. Here, we review findings that have led to the current models for dynamin function, either as a mechanochemical enzyme driving membrane fission or as a regulatory GTPase monitoring rate-limiting steps in CME. However, these models are not mutually exclusive and subsequent studies have provided evidence for both dynamin functions. Recent evidence derived from divergent in vivo and in vitro approaches suggests that dynamin plays a dual role in CME, functioning at early stages as a fidelity monitor to regulate clathrin-coated pit maturation and at later stages to directly catalyse membrane fission and clathrin-coated vesicle formation.

Figure 1. Domain structure and function of dynamin

Dynamin is a multidomain GTPase, containing an N-terminal GTPase domain, a middle domain, a pleckstrin homology (PH) domain, a GTPase effector domain (GED) and a C-terminal proline-arginine rich domain (PRD), whose functions are indicated. Dynamin exists in solution as a tetramer.

Figure 2. Two models for dynamin function in clathrin mediated endocytosis

(A) Model 1: Dynamin functions as a mechanochemical GTPase. GTP binding triggers dynamin assembly into collars and coordinated, assembly-stimulated GTP hydrolysis triggers a concerted conformation change to mediate fission. (B) Model 2: Dynamin functions as a regulatory GTPase. GTP-bound dynamin controls effectors that mediate vesicle formation. Self-assembly activates an internal GAP domain (GED) that negatively regulates dynamin function and terminates dynamin-effector interactions.

Figure 3. Dynamin plays a dual role in clathrin mediated function

(A) We propose a new reconciliatory model for dynamin function and suggest that dynamin plays a dual role in CME with sequentially function. Dynamin functions in early, rate-limiting stages of clathrin coated pit (CCP) maturation as a regulatory GTPase/fidelity monitor that receives input from SH3 domain-containing partners that monitor coat assembly, cargo concentration and curvature generation. Subsequently, dynamin functions as a self-limited, assembly-stimulated GTPase collar that catalyzes membrane fission. (B) We propose that self-assembly – the ‘switch’ between dynamin functional states – is regulated by SH3 domain-containing dynamin partners that sense critical events in CCV maturation. Through other partners, dynamin might also trigger late events in CME such as actin polymerization or uncoating.