Dynamic transport of SNARE proteins in the Golgi apparatus

Abstract

Localization of a membrane protein in a subcellular compartment can be achieved by its retention in the compartment or by its continuous transport toward this compartment. Previous results have suggested that specific enzymes are localized in the Golgi apparatus at least in part by selective retention and exclusion from transport vesicles. However, the function of some Golgi SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) proteins is not compatible with their exclusion from transport vesicles. To help understand the mechanism accounting for the localization of SNARE proteins in the Golgi apparatus, we analyzed their lateral distribution in the Golgi cisternae and their incorporation into transport vesicles. According to our results, all SNARE proteins are efficiently incorporated into transport vesicles, indicating that the localization of SNARE proteins in the Golgi apparatus is not based on a static retention mechanism. Detailed analysis suggested that incorporation into transport vesicles was more efficient for SNARE proteins restricted to the cis face of the Golgi as compared with SNAREs present at the trans face. Furthermore, overexpression of a cis-Golgi SNARE protein altered concomitantly its incorporation in transport vesicles and its intra-Golgi localization. These observations suggest that, contrary to resident Golgi enzymes, SNARE proteins are localized in the Golgi apparatus as the result of a dynamic transport equilibrium.cisternal maturation ͉ glycosyltransferase ͉ secretion ͉ vesicular transport ͉ membrane sorting

Molecular Mechanism of Mitotic Golgi Disassembly and Reassembly Revealed by a Defined Reconstitution Assay

“…SNARE proteins behave differently in the gradient upon different treatments. For example, Bet1 was found to be enriched in the vesicle fractions, regardless of the presence of mitotic kinases, consistent with the previously described data that it is a recycling SNARE protein in the cell (36,37). Gos28, a Golgi t-SNARE, was not found in the vesicles under interphase conditions, but it was slightly enriched in the vesicles in the presence of mitotic kinases.…”

Transmembrane domains control exclusion of membrane proteins from clathrin-coated pits

“…It is, however, probable that selective exclusion of membrane proteins occurs at every step of intracellular transport. During an earlier analysis of intra-Golgi transport, we indeed observed that resident Golgi proteins are less abundant at sites where vesicles form (the rims of cisternae), suggesting at least partial exclusion from transport vesicles (Cosson et al, 2002;Cosson et al, 2005;Orci et al, 2000). Similarly, we reported selective membrane exclusion during the formation of phagocytic or macropinocytic cups in Dictyostelium cells (Mercanti et al, 2006).…”

How the Golgi works: A cisternal progenitor model

“…The presence of GS15 on all cisternae permits this SNARE complex to mediate homotypic membrane fusion. Interestingly, although the SNAREs needed for endoplasmic reticulum to Golgi transport are localized exclusively at the rims of Golgi cisternae, GS15 and Syntaxin 5 are only 2-fold enriched at that location and are also seen in the middle of cisternae (38). Thus, the SNAREs proposed to mediate homotypic fusion are found precisely where they may be needed for more local fusion events.…”