Advances in vesicle trafficking of membrane proteins and their regulatory mechanisms

Abstract

<p indent="0mm">Membrane proteins are essential components of cell membrane structures and can be transported through vesicles as signaling molecules. The vesicle trafficking of membrane proteins is crucial for maintaining biological processes such as plant growth and development, substance exchange, cell recognition, immune response, and signal transduction. Both vesicle exocytosis and endocytosis play integral roles in regulating the activities and turnover of plasma membrane proteins required for signal triggering or attenuation at the cell surface. Exocytosis involves the transport of newly synthesized or recycled material to the plasma membrane (PM) in vesicles where they are released into the extracellular space. Recent investigations have shown that changing the balance between conventional and unconventional protein secretion can be an efficient strategy to respond to stressful conditions in eukaryotes. With the aid of an N-terminal located signal peptide (SP), conventional protein secretion delivers newly synthesized proteins from the endoplasmic reticulum (ER) to the Golgi apparatus and the <italic>trans</italic>-Golgi network (TGN) and finally to the PM or extracellular space via secretory vesicles or secretory granules. Unconventional protein secretion is responsible for the secretion of such SP-lacking protein. Notably, several unconventional protein secretion pathways mediated by distinct mechanisms have been reported in plants, most of which are related to stress responses. Conversely to exocytosis, endocytosis is the process by which material associated with the cellular surface is uptaken by invagination of the plasma membrane and internalized as an endocytic vesicle. In recent years, substantial progress has been made in the characterization of the plant components involved in clathrin-mediated and membrane microdomain-associated endocytic pathways. Moreover, the coordination of exocytic and endocytic trafficking has been shown to be important for various plant cell functions. An increasing pool of evidence has indicated that multiple cellular processes require tightly regulated exocytosis-endocytosis coupling, including nutrient uptake, signaling transduction, and plant–microbe interactions. However, compared to animal systems in which multiple mechanisms have been described, the functions and regulation of coupled trafficking processes in plants remain poorly understood. All proteins in the cells of an organism and most extracellular proteins are continually being degraded and replaced. The autophagy pathway is a conserved protein auto degradation pathway that delivers dysfunctional organelles or other cytoplasmic components to vacuoles for degradation and recycling in plants. In addition, autophagy is reported to be involved in plant development as part of processes such as root tip cell growth and differentiation. Recent observations have revealed possible interplay between vesicle trafficking and autophagic pathways in regulating vacuolar degradation in plants. In recent years, with constant advances in super-resolution microscopy and labeling methods, much progress has been made in mechanistic studies of membrane protein transportation, and a diversity of exocytic and endocytic trafficking pathways have been uncovered in plants. Although a variety of methods and techniques have been used to study plant vesicle trafficking, a systematic summary of vesicle trafficking pathways of plant membrane proteins has not been published. This review first introduces the relevant organelles involved in membrane protein vesicle trafficking, and then summarizes the different plant vesicle trafficking pathways for membrane proteins. Based on the review, we systematically consolidated the chemical treatments and mutants used in the study of plant vesicle trafficking. Finally, future directions for research in vesicle trafficking pathways of plant membrane proteins are discussed. With recent advancements, we expect major breakthroughs in our understanding of the regulatory mechanisms that determine the perception and adaptability of plants to their environments in the coming years.