Types of Trafficking at the Plasma Membrane

Protein trafficking at the plasma membrane (PM) involves (1) secretion of proteins to the PM or to the apoplast (exocytosis, secretion, or anterograde trafficking), (2) uptake of proteins at the PM for recycling or regulation of their activity (endocyto-sis or retrograde trafficking), and (3) moving proteins from one location on the PM to another (transcytosis). The integration of these processes is required for dynamic

Department of Horticulture and Landscape Architecture, Purdue University, 625 Agricultural Mall Drive, West Lafayette, IN 47907-2010, USA e-mail: [email protected]

A.S. Murphy et al. (eds.), The Plant Plasma Membrane, Plant Cell Monographs 19, DOI 10.1007/978-3-642-13431-9_2, © Springer-Verlag Berlin Heidelberg 2011

maintenance of cellular homeostasis following biotic or abiotic stimuli, such as herbivory, fungal infection, drought stress, or gravitropic or phototropic stimuli.

Secretion is the process by which proteins, lipids, and other molecules are trafficked, usually though the endoplasmic reticulum and Golgi-apparatus, and targeted to the plasma membrane (PM), extracellular space, or other organelles in the cytosol (Fig. 1). Endocytosis is the process by which nutrients, sterols, lipoproteins, peptide hormones, growth factors, and receptor-binding toxins are taken into cells. Endocytosis regulates the abundance and distribution of PM transport and receptor proteins, and it is an important mechanistic component of degradative and recycling mechanisms resulting in reuse of expensive transmembrane proteins and organelle homeostasis (Samaj et al. 2004). Transcytosis is well documented in animal systems where proteins undergo transcystic mediation of apical to basolat-eral redirection. Animals have gap junctions, which define the polarity of cells. Although the structural/mechanical basis of cellular polarity has not been elucidated in plants, transcytosis has been documented in plants cells: during embryogenesis, the PIN1 auxin efflux carrier is translocated from opposite sides of the cell via a guanine-nucleotide exchange factor for ADP-ribosylation factor GTPase-dependent transcytosis-like mechanism (Kleine-Vehn et al. 2008a).

Proteins that are targeted to or function at the PM are integral or peripheral membrane proteins. Integral membrane proteins have a membrane spanning helix

Fig. 1 Overview of trafficking pathway: ER to Golgi to TGN to PM; ER to PM. (a) simplified view of trafficking of proteins through the endomembrane system to and from the plasma membrane. Orange polygons show Golgi to PM traffic. Blue circles show Golgi to tonoplast (vacuolar membrane) traffic through the endomembrane system. Red ovals show a PM protein that is undergoing turnover in the vacuole. Green rectangles show a protein that is constitutively recycled. G Golgi, TGN trans-Golgi network, PM plasma membrane, EE early endosome, RE recycling endosome, MVB multivesicular body (prevacuolar compartment), V vacuole, CW cell wall. The nucleus, mitochondria, plastids, actin filaments, microtubules and endoplasmic reticu-lum have been omitted for clarity. Arrows indicate the direction of movement

Fig. 1 Overview of trafficking pathway: ER to Golgi to TGN to PM; ER to PM. (a) simplified view of trafficking of proteins through the endomembrane system to and from the plasma membrane. Orange polygons show Golgi to PM traffic. Blue circles show Golgi to tonoplast (vacuolar membrane) traffic through the endomembrane system. Red ovals show a PM protein that is undergoing turnover in the vacuole. Green rectangles show a protein that is constitutively recycled. G Golgi, TGN trans-Golgi network, PM plasma membrane, EE early endosome, RE recycling endosome, MVB multivesicular body (prevacuolar compartment), V vacuole, CW cell wall. The nucleus, mitochondria, plastids, actin filaments, microtubules and endoplasmic reticu-lum have been omitted for clarity. Arrows indicate the direction of movement or helices that anchor them to the PM, while peripheral membrane proteins are associated with the membrane through hydrophobic regions of the protein and may or may not be associated with integral membrane proteins. Initially, a protein that is targeted to the PM is synthesized on the rough endoplasmic reticulum (ER) (Fig. 1). From there, the protein may be trafficked to the cis-Golgi via COPII-coated vesicles (type I protein) or may be directly targeted to the PM (type II protein). From the cis-Golgi, the protein may be trafficked back to the rough ER via COPI-coated vesicles, or may continue through to the medial- and trans-Golgi compartments. If the protein requires modification for proper function, such as glycosylation by the glycosyl transferases resident in the Golgi stacks, then the protein may undergo multiple rounds of trafficking among the Golgi stacks via COPI-coated vesicles. From the trans-Golgi, the protein then traffics through the trans-Golgi network (TGN) via clathrin-coated vesicles (CCVs). From there, the TGN/early endosome compartment, the protein may traffic to another compartment or organelle or to the PM. In plants, the TGN and early endosomes (EE) appear to be one compartment in contrast to animals where discrete compartments (identified via markers) have been visualized (Lam et al. 2007, 2009). The TGN/EE compartment is the intersection of the secretory and endocytosis pathways, with proteins from the trans-Golgi, PM, prevacuolar compartment, and multivesicular bodies (Lam et al. 2007, 2009). Once at the PM, the protein may remain there or be trafficked back to the TGN/EE compartment through endocytosis via CCVs or receptor-mediated endocytosis. Once there, the protein may be sorted to return to the PM or be targeted to another compartment or organelle. The vesicles move on actin filaments or microtubules (composed of tubulin), which are protein scaffolds of cellular structure.

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