Membrane Trafficking: Intracellular Highways and Country Roads

Membrane trafficking, or the flow of membrane material between endomembrane compartments and the plasmalemma, is essential for transport of proteins and other macromolecules to various destinations inside and outside of the cell. Membrane trafficking also underlies the fundamental need for cells to maintain cellular homeostasis, as well as to meet specific demands during signal perception and transduction. The pathways of membrane protein trafficking, starting from the endoplasmic reticulum (ER), are long, branched, and occasionally even bidirectional. The blueprint of the endomembrane system is conserved among eukaryotes and comprises the ER, the Golgi apparatus, endosomes, and lytic compartments. The plant endomembrane system is apparently considerably more complex than in unicellular yeasts; the highly expanded protein repertoire devoted to the endomembrane system and vesicular processes inferred from plant genomes as well as its divergence from other kingdoms would argue for a system that has evolved to serve cellular strategies that best support the plant cell. The biosynthetic functions in the ER and Golgi are followed by sorting at the Golgi apparatus for antero-trafficking to the cell membrane or to lytic or storage vacuoles and retrograde trafficking back to the ER. Constitutive and signal-regulated endocytosis at the plasmalemma is followed by transport to the lytic vacuole or to the Golgi for sorting and recycling. Voluminous studies have been devoted in past decades to structurally and biochemically characterize the different endomembrane compartments and the processes each undertakes in overall cellular metabolism. Arrival of the post-genomic era, together with development of microscopic and computational tools in the last decade, has resulted in an explosion of studies, some reaffirming classical understanding, others uncovering new insights on the membrane trafficking machinery in plant cells as well as the cellular processes and whole plant-related phenomena that the endomembrane system supports. This Focus Issue is organized to provide both Updates and exciting new research articles on these recent findings and suggest paths to chart the future.

The Updates have been written by small teams of scientists who have impressive track records in their specific fields and they span a broad spectrum of topics central to membrane trafficking. The Staehelin and Kang (2008) Update reviews state-of-the-art, high-resolution capabilities in obtaining structural definitions in three dimensions for individual organelles and, in particular, reconstruction of organelles that are undergoing dynamic changes. This is followed by the Held et al. (2008) Update on the prowess of what combinations of fluorescence-based microscope and live cell imaging can achieve for spatial and functional assignments for specific molecules within the entire endomembrane system. The Rojo and Denecke (2008) and Robinson et al. (2008) Updates examine the most recently defined players in the secretory and endosomal pathways and how these may be used to interpret, reconcile, or extend prior results. These provide good roadmaps, especially for researchers whose work would benefit from a membrane trafficking perspective.

The involvement of a large number of regulatory molecules is discussed in the Updates on the small regulatory RAB and ARF GTPases by Nielsen et al. (2008), on SNAREs by Bassham and Blatt (2008), and on Rho GTPases by Yalovsky et al. (2008). Studies in these areas have advanced so much in the last few years that these Updates have gone far beyond a catalog of molecules and their potential functions and include vigorous discussions on the cellular, physiological, and developmental events that these molecules regulate, how they themselves are regulated, and how these regulatory pathways may integrate in the control of vesicular transport.

Several Updates deal with aspects of membrane trafficking from the point of view of a particular physiological process. Kwon et al. (2008) describe recent findings linking plant immunity to secretion pathways depositing compounds helping plants to counteract pathogens. Feraru and Friml (2008) focus on the acquisition of cell polarity by means of controlled deposition of membrane proteins, in particular the PIN auxin carriers. Here, specificity in the sorting of integral membrane proteins is evolving as a future topic of interest. Van Damme et al. (2008) discuss the connections between the cell cycle and membrane trafficking, while Geldner and Robatzek (2008) as well as Aker and de Vries (2008) highlight various aspects, including endocytosis and degradation, of plasma membrane receptors.

A large number of research manuscripts on different biological systems and based on a variety of approaches have been submitted to this Focus Issue. Among the articles in this issue are several studies based on root hairs and pollen tubes, polar growth cell types that are favorite model cell systems for studies in membrane trafficking. Monshausen et al. (2008) and Bove et al. (2008) highlight the use of imaging approaches combined with computational analysis to examine Ca²⁺ regulation of root hair growth and provide spatial definition of exocytic activity in pollen tubes, respectively. Yoo et al. (2008) emphasize the use of genetics to elucidate how a regulatory aspect of membrane trafficking affects root hair growth. Wang et al. (2008), using largely a transient expression approach, examine the involvement of an actin-binding protein on the regulation of actin and ion dynamics in elongating pollen tubes. Persia et al. (2008) report on biochemical studies that examine the localization of pollen tube sucrose synthase, a key enzyme for cell wall biosynthesis, and how phosphorylation may affect its association with the secretory pathway. The importance of cell wall biosynthesis is also highlighted in the article by Paredez et al. (2008), which examines the orientation and stability of the root cortical microtubules in mutants compromised in cellulose synthesis. The study of Kaneda et al. (2008) combines pulse-chase labeling and high-resolution electron microscopy to dissect the delivery pathway of monolignols to the extracellular matrix, which may be supported by specific membrane transporters rather than via Golgi-derived vesicles. This implicates diverse strategies in the delivery of biopolymers to the cell surface.

Several studies explore the functional connection between structural or regulatory components of endomembrane compartments and cellular or developmental processes. Li et al. (2008) describe an endosome-localized Ca²⁺-ATPase and its contribution to root growth. Konopka and Bednarek (2008) describe two dynamin-related proteins involved in plasma membrane maintenance in pollen and stigma cells. Esseling-Ozdoba et al. (2008) use synthetic fluorescent lipophilic vesicles to highlight the role of actin in establishing the cell plate in Tradescantia stamen hair cells. Lam et al. (2008) study inclusion of material into the developing cell plate that is secreted by the trans-Golgi network/early endosomal compartment rather then from the prevacuolar compartment/multivesicular bodies. Guo and Ho (2008) present a case for the involvement of the barley (Hordeum vulgare) aleurone HVA22 protein as a negative regulator of GA-mediated vacuolation/programmed cell death by means of inhibiting vesicular trafficking.

OUTLOOK

From the Updates as well as the research articles, it becomes evident that membrane trafficking is a very active and vigorous area of plant research. We hope that readers will find this collection of use as a teaching tool and an important resource for research in the area. Expected next thrusts in the field are to mine genomics and transcriptomics databases and to use state-of-the-art proteomics approaches to identify structural and regulatory components of the endomembrane system. Examples of this can be found in the research report of Kamei et al. (2008) on the combined use of bioinformatics and functional studies on PRA1, a membrane trafficking regulatory protein, and in the Groen et al. (2008) Update that reviews recent advanced proteomics to achieve more precise organellar assignment for proteins in the endomembrane system. What is emerging from a number of studies and Updates is that one simply cannot assume that plant counterparts of yeast or animal endomembrane proteins have the same function. Therefore, detailed genetic manipulations, biochemical analysis, and advanced imaging are indispensible to elucidate the precise biological role of the endomembrane system and its constituents.