Glycosylphosphatidylinositol (GPI) Modification Serves as a Primary Plasmodesmal Sorting Signal

• Published in: Plant physiology (Bethesda) 2016-10, Vol.172 (2), p.1061-1073
• Main Authors: Zavaliev, Raul, Dong, Xinnian, Epel, Bernard L.
• Format: Article
• Language: English
• Subjects: Arabidopsis - genetics; Arabidopsis - metabolism; Arabidopsis Proteins - genetics; Arabidopsis Proteins - metabolism; CELL BIOLOGY; Cell Membrane - metabolism; Cell Wall - metabolism; Glucan Endo-1,3-beta-D-Glucosidase - genetics; Glucan Endo-1,3-beta-D-Glucosidase - metabolism; Glycosylphosphatidylinositols - metabolism; Immunoblotting; Lipid-Linked Proteins - genetics; Lipid-Linked Proteins - metabolism; Membrane Glycoproteins - genetics; Membrane Glycoproteins - metabolism; Membrane Microdomains - metabolism; Microscopy, Confocal; Models, Biological; Plants, Genetically Modified; Plasmodesmata - genetics; Plasmodesmata - metabolism; Protein Transport - genetics
• ISSN: 0032-0889; 1532-2548
• DOI: 10.1104/pp.16.01026

Plasmodesmata (Pd) are membranous channels that serve as a major conduit for cell-to-cell communication in plants. The Pd-associated β-1,3-glucanase (BG_pap) and CALLOSE BINDING PROTEIN1 (PDCB1) were identified as key regulators of Pd conductivity. Both are predicted glycosylphosphatidylinositol-anchored proteins (GPI-APs) carrying a conserved GPI modification signal. However, the subcellular targeting mechanism of these proteins is unknown, particularly in the context of other GPI-APs not associated with Pd. Here, we conducted a comparative analysis of the subcellular targeting of the two Pd-resident and two unrelated non-Pd GPI-APs in Arabidopsis (Arabidopsis thaliana). We show that GPI modification is necessary and sufficient for delivering both BG_pap and PDCB1 to Pd. Moreover, the GPI modification signal from both Pd- and non-Pd GPI-APs is able to target a reporter protein to Pd, likely to plasma membrane microdomains enriched at Pd. As such, the GPI modification serves as a primary Pd sorting signal in plant cells. Interestingly, the ectodomain, a region that carries the functional domain in GPI-APs, in Pd-resident proteins further enhances Pd accumulation. However, in non-Pd GPI-APs, the ectodomain overrides the Pd targeting function of the GPI signal and determines a specific GPI-dependent non-Pd localization of these proteins at the plasma membrane and cell wall. Domain-swap analysis showed that the non-Pd localization is also dominant over the Pd-enhancing function mediated by a Pd ectodomain. In conclusion, our results indicate that segregation between Pd- and non-Pd GPI-APs occurs prior to Pd targeting, providing, to our knowledge, the first evidence of the mechanism of GPI-AP sorting in plants.