A global analysis of SNX27-retromer assembly and cargo specificity reveals a function in glucose and metal ion transport

Through this dual interaction SNX27 regulates the retrieval and recycling of hundreds of cargoes to the plasma membrane.

“Through this dual interaction SNX27 regulates the retrieval and recycling of hundreds of cargoes to the plasma membrane [26,29,31,58,59].”

Through its FERM domain, SNX27 may associate with cargoes containing a NPxY or NxxY sorting motif, with a preference for sequences phosphorylated at the tyrosine residue.

“Through its FERM domain, SNX27 may also associate with cargoes containing a NPxY or NxxY sorting motif (x refers to any residue), with SNX27 showing a preference for sequences that are phosphorylated at the tyrosine residue [29,60,61].”

The difference between the time scales of retromer knocksideways and the appearance of the GLUT1 missorting phenotype is consistent with the known rate of GLUT1 lysosomal-mediated degradation upon retromer suppression and reflects the relatively slow rate of endocytosis of this transporter.

“The difference between the time scales of retromer knocksideways (Fig. 1C–E) compared with the appearance of the GLUT1 missorting phenotype is entirely consistent with the known rate of GLUT1 lysosomal-mediated degradation observed upon retromer suppression and reflects the relatively slow rate of endocytosis of this transporter (Steinberg et al., 2013).”

Endosomal retrieval and recycling is orchestrated through a specialised endosomal nanodomain, the retrieval sub-domain.

“and is orchestrated through a specialised endosomal nanodomain, the retrieval sub-domain”

SNX27 regulates cargo recycling by engaging with the Retromer complex, a SNX-BAR dimer, and the WASH complex.

“SNX27 regulates cargo recycling by engaging with the Retromer complex (VPS35, VPS26A/B, and VPS29), a SNX–Bin/Amphiphysin/Rvs (BAR) dimer (SNX1/2 in complex with SNX5/6), and the WASH complex”

Through its PDZ domain, SNX27 can simultaneously interact with Retromer and cargoes containing a PDZ binding motif in their carboxy-terminal tail.

“Through its PDZ domain, SNX27 can simultaneously interact with Retromer and cargoes containing a PDZ binding motif (PDZbm) in their carboxy-terminal tail [26,29,31,57].”

Retromer associates with accessory proteins and cargo adaptors including sorting nexins, the WASH complex, RAB GTPase regulators including TBC1D5 and ANKRD27/VARP, and RME8.

“To function, Retromer associates with accessory proteins and cargo adaptors including sorting nexins (SNXs) [24][25][26][27][28][29][30][31][32][33][34], the WASH complex [35][36][37][38][39], RAB GTPase regulators including TBC1D5 and ANKRD27/VARP [40][41][42][43][44][45][46], and RME8 [47].”

VPS35 suppression decreased surface levels of GLUT1/SLC2A1, AMPA receptors GRIA2, GRIA3, GRIA4, NMDA receptors GRIN1, GRIN2b, and SLC4A7.

“Validating our approach, downregulated proteins included the wellestablished Retromer cargo glucose transporter-1 (GLUT1/SLC2A1) [29], the ionotropic glutamate AMPA receptors (GRIA2, GRIA3, GRIA4) and NMDA receptors (GRIN1, GRIN2b) [80][81][82][83][84][85], as well as many other previously identified proteins including the sodium bicarbonate cotransporter-3 (SLC4A7) (Figure 1D, Supplementary Table”

Endosomal cargo retrieval and recycling are orchestrated by several multi-protein assemblies including SNX27-Retromer and SNX3-Retromer, SNX17-Retriever and associated Commander super-assembly, the WASH complex and its regulatory MUST complex, and the SNX-BAR proteins and ESCPE-1.

“Orchestrating endosomal cargo retrieval and recycling are several multi-protein assemblies that include sorting nexin-27 (SNX27)-Retromer and SNX3-Retromer8–16, SNX17-Retriever and associated Commander super-assembly17–22, the branched F-actin polymerising WASH complex and its regulatory MAGE-L2/USP7/TRIM27 (MUST) complex23–25, and the SNX-BAR proteins and endosomal sorting complex required for ex”

The SNX27 PDZ domain directly interacts with C-terminal PDZ-binding motifs present in many neuronal surface proteins and is critical for the selective retrieval and recycling of SNX27-specific cargoes.

“This unique PDZ domain directly interacts with C-terminal PDZ-binding motifs present in many neuronal surface proteins and is critical for the selective retrieval and recycling of SNX27-specific cargoes”

It is unlikely that SNX27 is involved in LDLR trafficking.

“Our results and the findings of Steinberg et al.28 showing that the depletion of SNX27 in HeLa cells does not affect the levels of LDLR on the cell surface—as shown for other members of the LDLR family—indicate that it is unlikely that SNX27 is involved in LDLR trafficking.”

The study's data provide insight into the temporal dynamics of endosomal cargo sorting and support the established role of retromer in cell surface recycling.

“By visualising the sorting of endogenous GLUT1 and CI-MPR, our data provide insight into the temporal dynamics of endosomal cargo sorting and support the established role of retromer in cell surface recycling (Temkin et al., 2011; Steinberg et al., 2013).”

Besides its role in cargo retrieval and recycling within the endosomal pathway, retromer has emerged as a master regulator of RAB7 in nutrient sensing and signalling.

“Thus, besides its role in cargo retrieval and recycling within the endosomal pathway (Steinberg et al., 2013), retromer has emerged as a master regulator of RAB7 in nutrient sensing and signalling (Jimenez-Orgaz et al., 2018; Curnock et al., 2019; Kvainickas et al., 2019).”

Retromer orchestrates the retrieval of hundreds of internalised cargo and promotes their recycling to the cell surface, with GLUT1 as a prototypical cargo.

“The principal role of retromer is therefore is to orchestrate the retrieval of hundreds of internalised cargo and to promote their recycling to the cell surface (Temkin et al., 2011; Steinberg et al., 2013).”

The WASH complex functions in endocytic recycling, maintenance of endolysosomal network integrity, exocytosis, cell adhesion, and cell migration.

“The WASH complex functions in the endocytic recycling of various types of membrane proteins, maintenance of endolysosomal network integrity, exocytosis, cell adhesion, and cell migration (11-23).”