Zentrum für Molekularbiologie der Pflanzen (ZMBP)

Principles of vesicle trafficking

Membrane vesicles are formed on donor membranes and fuse with specific target membranes. Vesicle formation requires the activities of conserved protein families such as ARF guanine-nucleotide exchange factors (ARF-GEFs) and their substrates, ARF GTPases, as well as specific coat proteins and GTPase-activating proteins (ARF-GAPs) (Fig. 1a). Transport vesicles interact with their target membrane via RAB GTPases and tethering proteins. Subsequent membrane fusion is mediated by the formation of 4-helical trans-SNARE complexes between R-SNAREs (v-SNAREs or VAMPs) on the vesicle and Q-SNAREs or t-SNAREs (syntaxins/Qa-SNAREs and t-SNARE light chains/Qb,Qc-SNAREs or SNAP25/Qb,c-SNAREs) on the target membrane (Fig. 2).

Figure 1 ARF-GEFs and vesicle formation

(a) ARF GDP/GTP cycle. Cytosolic ARF-GDP is recruited to membrane by ARF-GEF mediating GDP-GTP exchange. ARF-GAP stimulated GTP hydrolysis leads to dissociation of ARF-GDP from the membrane.

Figure 2 Membrane fusion through SNARE complex formation

Disassembly of the cis-SNARE complex converts the open form of syntaxin to a closed form (5), which has to be reverted before trans-SNARE complex formation (1).