Protein transport in plant cells: in and out of the Golgi

In plant cells, the Golgi apparatus is the key organelle for polysaccharide and glycolipid synthesis, protein glycosylation and protein sorting towards various cellular compartments. Protein import from the endoplasmic reticulum (ER) is a highly dynamic process, and new data suggest that transport, at least of soluble proteins, occurs via bulk flow. In this Botanical Briefing, we review the latest data on ER/Golgi inter-relations and the models for transport between the two organelles. Whether vesicles are involved in this transport event or if direct ER-Golgi connections exist are questions that are open to discussion. Whereas the majority of proteins pass through the Golgi on their way to other cell destinations, either by vesicular shuttles or through maturation of cisternae from the cis- to the trans-face, a number of membrane proteins reside in the different Golgi cisternae. Experimental evidence suggests that the length of the transmembrane domain is of crucial importance for the retention of proteins within the Golgi. In non-dividing cells, protein transport out of the Golgi is either directed towards the plasma membrane/cell wall (secretion) or to the vacuolar system. The latter comprises the lytic vacuole and protein storage vacuoles. In general, transport to either of these from the Golgi depends on different sorting signals and receptors and is mediated by clathrin-coated and dense vesicles, respectively. Being at the heart of the secretory pathway, the Golgi (transiently) accommodates regulatory proteins of secretion (e.g. SNAREs and small GTPases), of which many have been cloned in plants over the last decade. In this context, we present a list of regulatory proteins, along with structural and processing proteins, that have been located to the Golgi and the 'trans-Golgi network' by microscopy.