Mechanisms of protein retention in the Golgi

Abstract

The protein composition of the Golgi is intimately linked to its structure and function. As the Golgi serves as the major protein-sorting hub for the secretory pathway, it faces the unique challenge of maintaining its protein composition in the face of constant influx and efflux of transient cargo proteins. Much of our understanding of how proteins are retained in the Golgi has come from studies on glycosylation enzymes, largely because of the compartment-specific distributions these proteins display. From these and other studies of Golgi membrane proteins, we now understand that a variety of retention mechanisms are employed, the majority of which involve the dynamic process of iterative rounds of retrograde and anterograde transport. Such mechanisms rely on protein conformation and amino acid-based sorting signals as well as on properties of transmembrane domains and their relationship with the unique lipid composition of the Golgi.

Figure 1.

The membrane topology of Golgi resident proteins. The majority of Golgi integral membrane proteins are type II; of these the retention mechanisms employed by the glycosyltransferases/glycosidases and by the SNAREs, have been best characterized. Peripheral membrane proteins may associate with Golgi membranes through protein–protein interactions, protein-lipid interactions or because of posttranslational modification (not depicted) see text for details. Representative proteins, which are highlighted in this article, are listed below their corresponding topology. For simplicity, only one representation of a multispanning membrane protein has been shown.

Figure 2.

Models and mechanisms of integral membrane protein retention in the Golgi. For simplicity, only a type II membrane protein is depicted; however, such retention strategies may apply to proteins of different topology as well as to multimembrane spanning proteins. Sorting signals may be either conformational or amino acid sequence-based (see text for further details). TMD (transmembrane domain). Figure adapted from Tu et al. (2010) and reprinted with the permission from Springer @ 2010.