In a recent study, the rice receptor for chitin, OsCERK1, was found to interact in rice protoplasts with HSP90 and the HOP/STI1 co-chaperones (Chen et al., ). Bimolecular fluorescence assays designated both the ER and PM as interaction sites, suggesting that the HOP/STI1-HSP90 complex is required for export of OsCERK1 from the ER, and may co-travel with OsCERK1 to the PM via the secretory pathway dependent on the small GTPase SAR1 (Chen et al., ). Hop/Sti1-RNAi mutants were more susceptible to a virulent strain of the rice blast fungus compared to controls, whereas overexpression of HOP/STI1 conferred resistance.
From the above studies, it is evident that ER occupies a key position among the membranous compartments of the secretory transport route. The first steps in receptor biogenesis including polypeptide folding, initial PTM modifications and the quality control mechanisms that ensure correct processing, are facilitated by proteins with diverse activities and are prerequisites for the guidance of new PM receptors out of the ER.
Whether such a regulated secretory process exists in plants to mediate the transport of PM-associated receptors is still an open question. Nevertheless, evidence accumulated over recent years argues for an important role of secretion in modulating the signaling functions of plant receptors. For instance, the steady-state levels of accumulation at the PM of one of the known tomato ethylene receptors (LeETR4) or of the Arabidopsis flagellin receptor (FLS2), correlated with the strength of their signaling output following activation by ligands. Suppression of LeETR4 expression by RNAi resulted in an early-ripening tomato fruit (Kevany et al., ); likewise, reduced FLS2 accumulation at the PM impaired the accumulation of reactive oxygen species (Boutrot et al., ; Mersmann et al., ). Moreover, quantification of the cellular levels of BRI1, brassinosteroid receptor (Li and Chory, ), revealed that BRI1 accumulated at the PM in an organ-dependent manner (Harter and Witthhoft, ; van Esse et al., ). Furthermore, a number of plant receptor-interacting proteins belonging to various classes have been discovered by recent work and shown to influence the biosynthesis, transport, and functional performance of receptors at the PM.
The anterograde transport of proteins is coupled with the process of protein maturation which occurs in the ER. Maturation includes the correct folding and the addition of post-translational modifications to newly synthesized polypeptides (Jurgens, ; Staehelin and Kang, ). Newly synthesized transmembrane proteins enter the secretory pathway at the ER, following translocation into the ER membrane via the translocon complex (Agarraberes and Dice, ; Rapoport, ). The ER proteome is enriched in molecular chaperones that promote the correct folding and glycosylation of imported proteins (Molinari and Helenius, ; Kleizen and Braakman, ). ER chaperones recruit nascent membrane proteins by binding to their exposed ER-retention domains, hydrophobic regions, or glycosylation motifs (Reddy and Corley, ; Smith et al., ). The recent work describing the role of several plant ER molecular chaperones in receptor maturation or export out of the ER is outlined below (Table ).
Given its inherent functions, the secretory pathway modulates the spatial and temporal distribution of a large fraction of cellular signaling elements, including receptors, co-receptors, and other associated components. It is generally accepted that plant PM-associated signaling receptors rely on the constitutive, generic secretory transport route to mature and reach destination sites on membranes (Rojo and Denecke, ; Cai et al., ). Nevertheless, the critical roles of the receptors in the cell along with the diversity in their functional and structural characteristics strongly argue for a high degree of flexibility in their biosynthesis and delivery to the activity sites. In yeast and animals, several factors under the control of the secretion process including the cellular amount of receptor molecules, intracellular compartmentalization, and the steady-state distribution and density of receptors at the PM, were found to govern the signaling specificity, amplitude, and the nature of the physiological output. Moreover, a highly regulated secretion process that guides the secretion of PM receptors was described in humans. This regulated secretion process was found to employ elements of the generic transport machinery but also to recruit additional receptor-specific components. The recruited proteins physically associate with nascent receptors and act as an interface between the receptor and the secretion machinery respond to relevant changes in the cellular environment. For example, events including the presence of ligands and accumulation of secondary messengers such as calcium ions, impact the efficiency of receptor secretion rate, and introduce additional checkpoints for its regulation. These and additional aspects of the PM receptor secretion in eukaryotic systems other than plants, have been described in numerous reviews (Mellman and Nelson, ; Cooray et al., ; Winckler and Mellman, ; Shilo and Schejter, ). It is clear that regardless of the receptors’ cellular function, structural characteristics or organism of origin, highly controlled and efficient pathways for the production and transport to the PM is a common requirement.
BiP, an Arabidopsis ER-localized chaperone from the heat shock protein (HSP) 70 family participates in the ER-associated quality control (ERQC) of BRI1 by interacting and retaining the defective BRI1-9 in the ER (Jin et al., ). Similarly, in rice OsBiP3 interacts with the PM receptor XA21 (Park et al., ). In BiP3 gain-of-function transgenic rice, both the stability and proteolytic cleavage of XA21 were impaired, compromising XA21-mediated immunity. Interestingly, although XA21 recognizes its ligand at the PM, the bulk of XA21 receptors expressed from the native promoter localized at the ER (Park et al., ), suggesting that XA21 accumulates at the ER to be readily released when needed. A similar strategy is used by human cells to sequester receptors in compartments of the secretory pathway for the “on-demand” delivery (Ascano et al., ). Since XA21 expressed from the 35S CaMV constitutive promoter localized at the PM (Chen et al., ), this possible regulatory step may be overridden by the XA21 overexpression.
Intracellular protein transport is emerging as critical in determining the outcome of receptor-activated signal transduction pathways. In plants, relatively little is known about the nature of the molecular components and mechanisms involved in coordinating receptor synthesis and transport to the cell surface. Recent advances in this field indicate that signaling pathways and intracellular transport machinery converge and coordinate to render receptors competent for signaling at their plasma membrane (PM) activity sites. The biogenesis and transport to the cell surface of signaling receptors appears to require both general trafficking and receptor-specific factors. Several molecular determinants, residing or associated with compartments of the secretory pathway and known to influence aspects in receptor biogenesis, are discussed and integrated into a predictive cooperative model for the functional expression of signaling receptors at the PM.
Intracellular transport of proteins involves a network of membranous compartments connected by transport vesicles and other transport structures. Both soluble proteins and proteins containing hydrophobic transmembrane domains rely on the general protein trafficking machinery for delivery via the secretory (also called “anterograde”) pathway to their final destinations. In plants, the endoplasmic reticulum (ER), Golgi complex, and trans-Golgi network (TGN) form the secretory transport conduit. In addition, a large number of organelle-associated proteins and lipids confer compartment identity, perform inter- and intra-compartment transport, and guide the trafficking of protein cargo (Carter et al., ; Jurgens, ; Dunkley et al., ). The transport of proteins within the plant secretory pathway is a dynamic and complex process. In the ER, membrane proteins destined for secretion accumulate in transport vesicles coated with the coat protein complex-II (COPII), which then bud from the ER-exit sites (ERESs). The COPII vesicles capture the mobile Golgi stacks from the cytosol and transfer their contents into the Golgi complex (Brandizzi et al., ; Runions et al., ; Staehelin and Kang, ). The transport factors that guide proteins from the ER to the Golgi return to the ER via coat protein complex-I (COPI)-coated vesicles (Stefano et al., ). As proteins progress through the Golgi, they undergo the final maturation steps and are subsequently transferred to the TGN, a sorting station which releases proteins to the PM (Staehelin and Kang, ). Targeted fusion of transport vesicles with the PM is facilitated by an octameric complex, the exocyst (Hala et al., ; Zhang et al., ).
In conclusion, Narita et al reveal the surprising ability of senescent cells to reorganize the morphology and the function of their endomembrane system. Their findings bring forth some exciting questions; in particular, what are the mechanism(s) that lead to formation of the TASCC? Are there senescence-associated gene expression programs that control membrane organization and dynamics? Are H-Ras and/or p53 directly involved? Answers to these questions are likely to shed light on the role of the TASCC in the maintenance of the senescent state and in tumor suppression.
Interestingly, the plant receptor-interacting proteins described above display several common characteristics with the animal RAPs. They (1) possess conserved protein–protein interaction motifs, (2) assemble in homo- and/or hetero-dimers, (3) influence the spatial distribution within organelles and/or at the PM, and (4) modulate the function of PM-associated signaling receptors. These characteristics suggest that plant receptor-interacting proteins link signaling components to the general trafficking machinery. It is likely that mechanisms similar to animals evolved in plants to modulate the selective transport of signaling elements to the cell surface. The synthesis, transport and display of functional signaling receptors at the PM appears as a tightly regulated process that relies on components of the general transport machinery and receptor-specific elements recruited along the secretory route.