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

Abstract

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.

Fig. 1. Transmission electron micrographs of Golgi stacks in tobacco (A) and maize roots (B). A, Cross‐section of a Golgi stack in a tobacco root cap cell. High‐pressure freezing and freeze‐substitution improves the ultrastructural preservation of intercisternal filaments towards the trans‐face of the Golgi stack (arrows). B, Face view of a Golgi cisternum in a maize root meristematic cell. Zinc iodide and osmium tetroxide impregnation selectively stains the ER and the Golgi and clearly shows the fenestrated margins of the Golgi cisternum. ER, Endoplasmic reticulum; M, mitochondrion; V, vesicle. Bars = 200 nm.

Fig. 2. Confocal laser scanning micrographs showing the spatial relationship between Golgi stacks and ER (A) and between Golgi stacks and actin filaments (B). A, 3D‐reconstruction (Velocity®) of serial optical sections through the cortical cytoplasm of a tobacco leaf epidermal cell transiently transformed with a GFP‐fusion targeted to the ER (GFP‐HDEL in green) and a YFP‐fusion labelling the Golgi (ST‐YFP in red). Golgi stacks are in close association with the ER network. B, Optical section through the cortical cytoplasm of a tobacco BY2 cell stably transformed with ST‐GFP (in green). Affinity labelling of actin by rhodamine‐phalloidine (in red) reveals that Golgi stacks are aligned with actin filaments (arrows). Bars = 10 µm.

Fig. 3. Models of ER‐to‐Golgi protein transport. A, The ‘vacuum cleaner model’ (Boevink et al., 1998) suggests that Golgi stacks move over the ER constantly picking up cargo. According to this model, the whole ER surface is capable of forming export sites, resulting in their random distribution. In contrast, the ‘stop‐and‐go’ model (B) hypothesizes that Golgi stacks stop at fixed ER export sites to take up cargo from the ER, before moving onto the next stop. In the more dynamic ‘mobile export sites’ model (C), Golgi stacks and ER export sites move together as ‘secretory units’ (Brandizzi et al., 2002b) allowing cargo to be transported from the ER towards the Golgi at any time during movement.

Fig. 4. Confocal laser scanning micrographs showing the location of different regulatory and structural proteins of the secretory pathway in relation to Golgi stacks. A, Optical section through the cortical cytoplasm of a tobacco BY2 cell stably transformed with the Golgi marker GmMan1‐GFP (Glycine max α‐mannosidase 1‐GFP, green channel) after fixation and immunolocalization with anti‐AtArf1p antibodies (red channel). The merged image clearly reveals that anti‐AtArf1p labelling is associated with the Golgi, forming a ring‐shaped pattern confined to the periphery of each stack (Ritzenthaler et al., 2002). B, Optical section through the centre of a transgenic GmMan1‐GFP BY2 cell after fixation (GFP signal in green channel) and immunolocalization with anti‐Atγ‐COP antibodies (red channel). As can be seen in the merged image, the COPI coatomer subunit co‐localizes with Golgi‐associated GFP‐fluorescence. As for Arf1p, anti‐Atγ‐COP fluorescence is restricted to the margins of the Golgi stacks (Ritzenthaler et al., 2002). C, Projection of 15 optical sections (1 µm per section) through a pea root tip cell after fixation and double‐immunolabelling with anti‐VSR antibodies (17F9, green channel) and JIM 84, a trans‐Golgi marker (red channel). As shown in the merged image, more than 90 % of the prevacuolar organelles labelled by 17F9 are separate from Golgi stacks (Li et al., 2002). Occasionally, fluorescence labelling by the two antibodies co‐localizes (merged image, open arrow). D, Optical section through the cortical cytoplasm of a leaf epidermal cell of a transgenic ST‐GFP tobacco plant (GFP signal in green channel) transiently transformed with YFP‐AtRab2a (YFP signal in red channel). As can be seen in the merged image, both fluorescent fusion proteins co‐localize in Golgi stacks. In addition to the Golgi, YFP‐AtRab2a labels small spherical structures (sometimes measuring up to 3 µm in diameter) in which no GFP signal can be detected (merged image, arrows). Bars = 5 µm (A–C, insert D) and 20 µm (D). Micrographs for A and B kindly provided by Christophe Ritzenthaler and that for C by Liwen Jiang.