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. 2018 Feb 5;217(2):585–599. doi: 10.1083/jcb.201706135

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© 2018 Liu et al.

This article is distributed under the terms of an Attribution–Noncommercial–Share Alike–No Mirror Sites license for the first six months after the publication date (see http://www.rupress.org/terms/). After six months it is available under a Creative Commons License (Attribution–Noncommercial–Share Alike 4.0 International license, as described at https://creativecommons.org/licenses/by-nc-sa/4.0/).

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Figure 9. — Summary models for folding and processing of GPI-APs regulated by calnexin and ER homeostasis. Top: under normal conditions, N-glycans and GPI are transferred to newly synthesized GPI-APs. The glucose residues on N-glycans are trimmed by α-glucosidases I and II, resulting in a Glc1Man9GlcNAc2 structure, which is recognized by calnexin. Calnexin assists protein folding and facilitates GPI-inositol deacylation by PGAP1 through the temporal ER retention of GPI-APs. Calnexin also interacts with PGAP1 to increase the efficiency of the GPI-inositol deacylation. Once the protein portion of each GPI-AP is folded, calnexin dissociates from the GPI-AP, which is further remodeled by PGAP5 and efficiently incorporated into transport vesicles by a signal-mediated pathway involving p24 proteins. Middle: in CANX-KO cells, GPI-APs cannot retain in the ER and decrease the efficient interaction with PGAP1, resulting in transport of some GPI-APs without processing. Calnexin-independent GPI-inositol deacylation also exists. Bottom: under chronic ER stress conditions, misfolded GPI-APs accumulate in the ER and occupy the calnexin and PGAP1. Availability of calnexin and PGAP1 is therefore decreased, and normal GPI-APs without processing leak into transport vesicles through a bulk flow pathway, resulting in expression of inositol-acylated GPI-APs on the cell surface.