Unconventional Protein Secretion in Plants

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. The large numbers of LSPs in the plant secretome suggest that UPS also occurs in plants. Although the direct transport of LSPs across the plant plasma membrane (PM) has not yet been described, there is a good probability 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 sRNAs, that are passively taken up from the cytosol into the intraluminal vesicles of MVBs, can 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. Expected final online publication date for the Ann...