Regulating cytoskeleton-based vesicle motility

Abstract

During vesicular transport, the assembly of the coat complexes and the selection of cargo proteins must be coordinated with the subsequent translocation of vesicles from the donor to an acceptor compartment. Here, we review recent progress toward uncovering the molecular mechanisms that connect transport vesicles to the protein machinery responsible for cytoskeleton-mediated motility. An emerging theme is that vesicle cargo proteins, either directly or through binding interactions with coat proteins, are able to influence cytoskeletal dynamics and motor protein function. Hence, a vesicle's cargo composition may help direct its intracellular motility and targeting.

Figure 1. Motor protein-mediated motility of organelles and transport vesicles requires spatial, temporal, and directional regulation

(A) Mechanisms for spatial regulation must exist to ensure that while a nascent vesicle binds to a motor and becomes motile, the donor organelle does not. (B) Temporal regulation should occur in order to coordinate motor recruitment with other steps in vesicle formation. Motor-based motility should be blocked until vesicle coat assembly, cargo packaging, and vesicle scission have been completed (C) In the cases where vesicles or cargo are recycled or undergo bidirectional transport, regulatory processes must ensure that the correct motor is functional. For example, during recycling between the ER and the Golgi apparatus coatomer-coated vesiculotubular clusters undergo dynein-mediated anterograde transport, whereas coatomer-coated COPI vesicles mediating retrograde transport from the Golgi apparatus utilize kinesin motors.

Figure 2. Some activated G-protein coupled receptors undergo delayed internalization by directing interactions between clathrin-coated vesicles and the actin cytoskeleton

G-protein coupled receptors (GPCRs) are recruited to clathrin-coated pits after ligand binding (Left). For GPCRs that contain PDZ-ligand domains, an anchor between the forming vesicle and the actin cytoskeleton may be formed via PDZ-domain-containing proteins and the ERM-family of actin binding proteins. This creates clathrin-coated pits with delayed internalization and extended cell-surface residence times [22]. The transferrin receptor (Right) is also internalized through clathrin-coated pits. However, transferrin lacks a PDZ domain and does not form ERM-dependent interactions with cortical actin. Thus for transferring containing vesicles, the scission and internalization steps proceed rapidly.

Figure 3. Actin dynamics and dynein recruitment on COPI vesicles is regulated through a cargo-sensitive binding interaction between coatomer and Cdc42

Actin polymerization on Golgi vesicles is stimulated by the ARF1-dependent recruitment of a complex between the COPI-coat protein, coatomer and the Rho-family GTP-binding protein, Cdc42 [35]. ARF1-dependent Cdc42 function at the Golgi apparatus is specifically regulated by the GTPase activating protein, ARHGAP10 [39]. Active Cdc42 also inhibits dynein recruitment in an actin-dependent manner [36]. Coatomer cannot be simultaneously bound to Cdc42 and the p23 putative cargo receptor. Thus, the presence of p23 acts to block actin polymerization and stimulate dynein recruitment. This signaling may ensure that vesicle motility does not commence until the completion of vesicle assembly and cargo packaging.

Figure 4. Cargo binding to the globular tail domain of myosin 5a regulates its structure and activity

(A) The heavy chain of myosin 5 contains an N-terminal head domain and a C-terminal globular tail domain. The head domain contains the binding sites for actin and ATP. The globular tail domain contains the cargo-binding sites. Structural analysis of the yeast myosin 5 protein, Myo2p, reveals that the globular tail contains two distinct cargo-binding modules one for transport vesicles and a second for vacuoles [45,46]. (B) The globular tail of myosin 5a can bind and inhibit the ATPase activity of the N-terminal head domain. It is proposed that cargo binding to the globular tail domain of myosin 5 causes a conformation change leading to the activation of the motor [50,51].