Protein secretion in plants: conventional and unconventional pathways and new techniques

Abstract

Protein secretion is an essential process in all eukaryotic cells and its mechanisms have been extensively studied. Proteins with an N-terminal leading sequence or transmembrane domain are delivered through the conventional protein secretion (CPS) pathway from the endoplasmic reticulum (ER) to the Golgi apparatus. This feature is conserved in yeast, animals, and plants. In contrast, the transport of leaderless secretory proteins (LSPs) from the cytosol to the cell exterior is accomplished via the unconventional protein secretion (UPS) pathway. So far, the CPS pathway has been well characterized in plants, with several recent studies providing new information about the regulatory mechanisms involved. On the other hand, studies on UPS pathways in plants remain descriptive, although a connection between UPS and the plant defense response is becoming more and more apparent. In this review, we present an update on CPS and UPS. With the emergence of new techniques, a more comprehensive understanding of protein secretion in plants can be expected in the future.

Cell polarity: Regulators and mechanisms in plants

“…With regard to polarly localized proteins, they can be integral to the plasma membrane (PM) or associated with the PM. For the integral membrane proteins, to reach the PM, they are first synthesized in the endoplasmic reticulum (ER), followed by vesicle delivery along the secretory pathway through the Golgi apparatus and the trans‐Golgi network (TGN), and finally reach the PM by exocytosis and vesicle fusion (Wang et al 2017b). Many proteins are dynamically regulated at the plasma membrane where they play their biological function, while are also endocytosed via the clathrine‐dependent and/or ‐independent pathways (Chen et al 2011; Zhang et al 2019).…”

The vesicular trafficking system component MIN7 is required for minimizing Fusarium graminearum infection

“…This warrants further examination in a follow-on study, as well as analysis of response to F. graminearum in additional vesicular trafficking gene mutants that were not investigated in the current study and including those whose association with host immunity has not yet been demonstrated. It would also be interesting to investigate contribution of unconventional secretion pathways related to pathogen attack or stress conditions (Wang et al, 2018), which involve direct fusion of the central vacuole, multivesicular bodies or EXPO (a double-membrane exocyst-positive organelle) with the PM, to plant interactions with F. graminearum . Particularly interesting in this respect are the exosomes, small membranous vesicles that are thought to be released to the apoplast following fusion of MVBs with the PM (Rutter and Innes, 2017).…”

Extracellular vesicles: a missing component in plant cell wall remodeling

“…UPS pathways can also liberate proteins into the extracellular compartment using different mechanisms, some of them involving EV. Although EV biogenesis in plants is still unknown, proposed pathways include EXPO fusion or MVB fusion to the plasmalemma ( Wang et al , 2017 ). These mechanisms would be able to release vesicles into the apoplast that can directly interact with cell wall components or, alternatively, plant EV could also burst or break in the extracellular compartment to release their contents and exert their activity.…”