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Protoplasma

, Volume 253, Issue 1, pp 31–43 | Cite as

Unconventional protein secretion in plants: a critical assessment

  • David G. RobinsonEmail author
  • Yu Ding
  • Liwen Jiang
Review Article

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.

Keywords

Exocyst Exocyst-positive organelle (EXPO) Exosome Leaderless secretory proteins (LSPs) Multivesicular body (MVB) Plant secretome Unconventional protein secretion (UPS) 

Abbreviations

Acb1

Acyl-CoA-binding protein 1

BFA

Brefeldin A

CPS

Conventional protein secretion

CUPS

Compartment of unconventional protein secretion

EHM

Extrahaustorial membrane

ER

Endoplasmic reticulum

ESCRT

Endosomal sorting complex required for transport

EXPO

Exocyst-positive organelle

ILV

Intraluminal vesicle

LSP

Leaderless secretory protein

MVB

Multivesicular body

PM

Plasma membrane

SNARE

Soluble NSF attachment protein receptor

TGN

Trans-Golgi network

UPS

Unconventional protein secretion

Notes

Acknowledgments

Our research has been supported by grants from the Research Grants Council of Hong Kong (CUHK465112, CUHK466613, CUHK2/CRF/11G, C4011-14R, and AoE/M-05/12), the National Natural Science Foundation of China (31470294), the CAS-Croucher Funding Scheme for Joint Laboratories, and the Shenzhen Peacock Project (KQTD201101).

Conflict of interest

The authors declare that they have no conflict of interest.

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© Springer-Verlag Wien 2015

Authors and Affiliations

  1. 1.Centre for Organismal StudiesUniversity of HeidelbergHeidelbergGermany
  2. 2.Centre for Cell & Developmental Biology and State Key Laboratory of AgrobiotechnologyThe Chinese University of Hong KongHong KongChina

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