Apoplastic exosome-like vesicles: A new way of protein secretion in plants?

Abstract

These authors contributed equally to this work.Keywords: apoplast, protein secretion, signal sequence, non-classical secretionThe presence of apoplastic proteins without predicted signal peptide in the gene sequence suggests the existence of protein secretion independent of the ER/Golgi classical route. In animals, one of the pathways proposed for alternative protein secretion involves the release of exosomes to the extracellular space. Although this pathway has not been dissected in plants some indirect evidence is emerging. We have reported that apoplastic fractions of sunflower seeds contain exosome-like vesicles. Besides, these vesicles are enriched in the lectin Helja, which is immunolocalized in the extracellular space even if it the protein has no predicted signal peptide. Here we show that Helja is not glycosylated and its secretion is insensitive to brefeldin A, two of the major characteristics to discard ER/Golgi-mediated protein transport. Moreover, the levels of Helja in sunflower extracellular vesicles are not affected by brefeldin A treatment. Our results suggest that Helja could be exported through an exosome-mediated pathway and point out that this mechanism may be responsible for the secretion of at least part of the leaderless proteins detected in the extracellular compartment of plants.

Molecular characterization of Helja, an extracellular jacalin-related protein from Helianthus annuus: Insights into the relationship of this protein with unconventionally secreted lectins

“…Significantly, no N-terminal hydrophobic peptide could be detected using the SignalP in silico tool. Thus, these data suggest that the primary translation product encoding Helja lacks the signal peptide, consistent with the hypothesis regarding the unconventional sorting of this protein Regente et al, 2012). Additionally, the Helja protein sequence lacks the KKKK stretch characteristic of nuclear localization, a feature associated with some members of the jacalin family (Lannoo and Van Damme, 2010).…”

“…Several lines of evidence, notably immunolocalization experiments through confocal microscopy, have demonstrated that Helja is located in the apoplast of sunflower seeds. Nevertheless, the N-terminal signal peptide that drives the proteins to the secretory pathway is absent in the genomic sequence of this protein Regente et al, 2012). This feature, together with the fact that the protein is not glycosylated and its secretion is insensitive to brefeldin A , suggests that Helja might be an unconventionally secreted lectin.…”

Exosomes: mediators of communication in eukaryotes

“…For example, they are secreted by protists like Dictyostelium discoideum (Lavialle et al, 2009) and Trypanosoma cruzi (Bayer-Santos et al, 2012), fungi (Rodrigues et al, 2011), plants (Regente et al, 2012), and animals from invertebrates such as Drosophila melanogaster (Korkut et al, 2009;Beckett et al, 2012;Gross et al, 2012) and Caenorhabditis elegans (Liegeois et al, 2006) to vertebrates (Record et al, 2011).…”

Routes to and from the plasma membrane: bulk flow versus signal mediated endocytosis

“…In contrast membrane proteins remain within the pathway, unless they segregate into the intraluminal vesicles of post Golgi organelles to reach either the vacuole lumen for degradation 1 or to be secreted as "exosomes." [2][3][4] While in the pathway, membrane proteins visit and recycle between many compartments, and they thus need a multitude of signals to regulate their steady state levels.…”