Glycosylphosphatidylinositol (GPI) modification serves as a primary plasmodesmal targeting signal

Abstract

Plasmodesmata (Pd) are membranous channels that serve as a major conduit for cell-to-cell communication in plants. The Pdassociated b-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. Domainswap 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.

Plant glycosylphosphatidylinositol anchored proteins at the plasma membrane‐cell wall nexus

“…The sterol composition of PD was shown to be important for localization of two GPI‐anchored PD proteins, Callose Binding 1 (PDCB1) and β ‐1,3‐glucanase (PdBG2) (Grison et al ). In addition, delivery of PDCBP1 and PdBGs to PD was dependent on a GPI anchor (Grison et al ; Zavaliev et al ). These studies suggest that the specific membrane composition of PD is required for targeted secretion of GAPs to these sites.…”

“…Interestingly, fusion of only the GPI anchors from PDCBP1 and pdBG2, as well as the GPI anchors from two non‐PD localized proteins, AGP4 and LTGP1 to reporter proteins resulted in their enrichment in PDs. In non‐PD located GAPs, the other functional domains within the protein are proposed to override the PD targeting (Zavaliev et al ). This is reminiscent of the apical/basolateral sorting in epithelial cells and supports the notion that GPI anchors act as a preliminary sorting signal during secretion and in the GA/TGN other features in the protein either reinforce or redirect microdomain location.…”

Multiple C2 domains and transmembrane region proteins ( MCTP s) tether membranes at plasmodesmata

“…They contain a strand of ER, continuous through the pores, tethered extremely tightly (~10 nm) to the PM by spoke-like elements [24,25] whose function and identity are unknown. Inside plasmodesmata, specialised subdomains of the ER and the PM co-exist, each being characterised by a unique set of lipids and proteins, both critical for proper function [6,12,[26][27][28][29][30][31]. Where it enters the pores, the ER becomes constricted to a 15-nm tube (the desmotubule) leaving little room for lumenal trafficking.…”

Identification of Plasmodesmal Localization Sequences in Proteins <em>In Planta</em>

“…The third sequence is from the PDLP1 plasmodesmata-resident type 1 membrane protein, and it is represented by a transmembrane domain 16 . Finally, the fourth Pd targeting sequence was recently reported for glycosylphosphatidylinositol (GPI)-anchored proteins and it is represented by the glycosylphosphatidylinositol (GPI) modification signal 17 .…”