, 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


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.


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



Acyl-CoA-binding protein 1


Brefeldin A


Conventional protein secretion


Compartment of unconventional protein secretion


Extrahaustorial membrane


Endoplasmic reticulum


Endosomal sorting complex required for transport


Exocyst-positive organelle


Intraluminal vesicle


Leaderless secretory protein


Multivesicular body


Plasma membrane


Soluble NSF attachment protein receptor


Trans-Golgi network


Unconventional protein secretion



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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