Dual Independent Roles of the p24 Complex in Selectivity of Secretory Cargo Export from the Endoplasmic Reticulum

Abstract

The cellular mechanisms that ensure the selectivity and fidelity of secretory cargo protein transport from the endoplasmic reticulum (ER) to the Golgi are still not well understood. The p24 protein complex acts as a specific cargo receptor for GPI-anchored proteins by facilitating their ER exit through a specialized export pathway in yeast. In parallel, the p24 complex can also exit the ER using the general pathway that exports the rest of secretory proteins with their respective cargo receptors. Here, we show biochemically that the p24 complex associates at the ER with other cargo receptors in a COPII-dependent manner, forming high-molecular weight multireceptor complexes. Furthermore, live cell imaging analysis reveals that the p24 complex is required to retain in the ER secretory cargos when their specific receptors are absent. This requirement does not involve neither the unfolded protein response nor the retrograde transport from the Golgi. Our results suggest that, in addition to its role as a cargo receptor in the specialized GPI-anchored protein pathway, the p24 complex also plays an independent role in secretory cargo selectivity during its exit through the general ER export pathway, preventing the non-selective bulk flow of native secretory cargos. This mechanism would ensure receptor-regulated cargo transport, providing an additional layer of regulation of secretory cargo selectivity during ER export.

MED27, SLC6A7, and MPPE1 Variants in a Complex Neurodevelopmental Disorder with Severe Dystonia

“…MPPE1 encodes PGAP5, a widely expressed metalloproteinase 19 integral to the GPI biosynthesis and protein‐anchoring pathway, a highly conserved eukaryotic posttranslational modification 10 . The addition and modification of a GPI anchor is essential for the trafficking of certain proteins from the ER to the Golgi and cell surface 20,21 . The MPPE1 variant p.Arg329* likely leads to translation of a truncated protein lacking the KxKxx ER retrieval signal, which is crucial for correct PGAP5 localization 7 .…”

“…10 The addition and modification of a GPI anchor is essential for the trafficking of certain proteins from the ER to the Golgi and cell surface. 20,21 The MPPE1 variant FIG. 1.…”

“… 10 The addition and modification of a GPI anchor is essential for the trafficking of certain proteins from the ER to the Golgi and cell surface. 20 , 21 The MPPE1 variant p.Arg329* likely leads to translation of a truncated protein lacking the KxKxx ER retrieval signal, which is crucial for correct PGAP5 localization. 7 We postulate that this would reduce cell‐surface expression of GPI‐APs (FLAER and CD73).…”

p24-Tango1 interactions ensure ER-Golgi interface stability and efficient transport

“…Nevertheless, along with reports of defects in the transport of specific cargos, evidence of more general impairments upon p24 deficiencies exist in the literature as well, such as altered Golgi-ER retrograde transport (Aguilera-Romero et al, 2008; Gommel et al, 2001; Majoul et al, 1998; Montesinos et al, 2014) and abnormal Golgi morphology (D’Arcangelo et al, 2015; Denzel et al, 2000; Koegler et al, 2010; Lavoie et al, 1999; Mitrovic et al, 2008; Pastor-Cantizano et al, 2018; Rojo et al, 2000). Moreover, broad roles have been ascribed to p24 proteins in mediating ER retention for quality control (Belden and Barlowe, 2001; Dvela-Levitt et al, 2019; Gomez-Navarro et al, 2020; Lopez et al, 2020; Ma et al, 2017; Springer et al, 2000; Wen and Greenwald, 1999), membrane contact during autophagosome formation from the ERGIC (ER Golgi Intermediate Compartment) (Li et al, 2022) and lipid transfer between the ER and Golgi (Anwar et al, 2022). Fitting all these proposed functions, cargo-specific and general, into a consistent view is problematic.…”

Crosslinking assay to study a specific cargo-coat interaction through a transmembrane receptor in the secretory pathway

“…Due to this luminal topology, newly synthesized GPI-APs require a transmembrane cargo receptor that connects them with the cytosolic COPII coat for ER export. Previous studies from our laboratory and others suggested that the p24 protein complex, composed of type I transmembrane p24 proteins, can act as a specific cargo receptor for GPI-APs [7][8][9]. It was hypothesized that the p24 receptor would link GPI-APs to Lst1, a specialized isoform of the major COPII coat cargo binding subunit Sec24, to be actively packaged into nascent COPII vesicles [5,7,10].…”