title: Unconventional protein secretion in plants: a critical assessment.
abstract: Unconventional protein secretion (UPS) is a collective term for mechanisms by which cytosolic proteins that lack a signal peptide ("leaderless secretory proteins" (LSPs)) can gain access to the cell exterior. Numerous examples of UPS have been well documented in animal and yeast cells. In contrast, our understanding of the mechanism(s) and function of UPS in plants is very limited. This review evaluates the available literature on this subject. The apparent large numbers of LSPs in the plant secretome suggest that UPS also occurs in plants but is not a proof. Although the direct transport of LSPs across the plant plasma membrane (PM) has not yet been described, it is possible that as in other eukaryotes, exosomes may be released from plant cells through fusion of multivesicular bodies (MVBs) with the PM. In this way, LSPs, but also small RNAs (sRNAs), that are passively taken up from the cytosol into the intraluminal vesicles of MVBs, could reach the apoplast. Another possible mechanism is the recently discovered exocyst-positive organelle (EXPO), a double-membrane-bound compartment, distinct from autophagosomes, which appears to sequester LSPs.
authors.affiliation: Centre for Organismal Studies, University of Heidelberg, Im Neuenheimer Feld 230, D-69120, Heidelberg, Germany. david.robinson@urz.uni-heidelberg.de.
authors.forename: David G
authors.initials: DG
authors.lastname: Robinson
authors.affiliation: Centre for Cell & Developmental Biology and State Key Laboratory of Agrobiotechnology, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong, China.
authors.forename: Yu
authors.initials: Y
authors.lastname: Ding
authors.affiliation: Centre for Cell & Developmental Biology and State Key Laboratory of Agrobiotechnology, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong, China.
authors.forename: Liwen
authors.initials: L
authors.lastname: Jiang
mesh_terms: D000818:Animals; D055354:Exosomes / Q000378:metabolism; D054884:Host-Pathogen Interactions; D008322:Mammals / Q000378:metabolism; D010940:Plant Proteins / Q000378:metabolism*; D010944:Plants / Q000378:metabolism*; D012441:Saccharomyces cerevisiae / Q000378:metabolism
chemical_list: D010940:Plant Proteins
keywords: Exocyst; Exocyst-positive organelle (EXPO); Exosome; Leaderless secretory proteins (LSPs); Multivesicular body (MVB); Plant secretome; Unconventional protein secretion (UPS)