SNX2

SNX2 is a sorting nexin that organizes endosomal retrograde trafficking as a membrane-recruitment module of the mammalian retromer pathway (PMID:17101778). It assembles into an alternative SNX1/SNX2 heterodimer (or homodimer) whose presence on endosomes is required to recruit the Vps26-Vps29-Vps35 heterotrimer, while SNX1/2 themselves localize to endosomes independently of that subcomplex; either SNX1 or SNX2 suffices to retrieve the cation-independent mannose 6-phosphate receptor to the trans-Golgi network (PMID:17101778). SNX1 and SNX2 act with partial redundancy in retrograde transport, as combined depletion blocks Shiga toxin delivery to the Golgi by ~80% whereas single depletion gives ≥40% inhibition (PMID:17498660). Beyond retrieval, SNX2 cooperates with SNX1 to tubulate early endosomes toward VAPB-positive ER subdomains, building endosome-ER contact sites that support autophagosome biogenesis during starvation (PMID:36585258). As part of the ESCPE-1 (SNX2/SNX6) complex, SNX2 deforms cargo-motif- and lipid-enriched membranes in a reconstituted system, with VARP required to assemble a larger endosomal supercomplex linking SNX27, ESCPE-1, and Retromer [PMID:bio_10.1101_2024.07.11.603126].

1. 2001 Low

   Before its endosomal role was defined, an unbiased interaction screen placed SNX2 in a protein network, linking it to FBP17 and the EGF receptor pathway.

   Evidence Yeast two-hybrid screen of a human kidney library

   PMID:11438682

   Open questions at the time

   • Single yeast two-hybrid hit with no in-cell or in vitro validation
   • Functional consequence of the FBP17 interaction not established
   • No link drawn to retromer function
2. 2006 High

   Established the architectural logic of mammalian retromer by showing SNX1/SNX2 form an autonomous membrane-recruitment dimer required to bring the Vps26-Vps29-Vps35 trimer to endosomes for CI-MPR retrieval.

   Evidence Biochemical fractionation, reciprocal Co-IP, siRNA knockdown and CI-MPR trafficking assays in HeLa cells

   PMID:17101778

   Open questions at the time

   • Did not resolve whether SNX1 and SNX2 have distinct cargo preferences
   • Lipid/PI3P-binding determinants of recruitment not dissected here
   • Structural basis of SNX1/2 dimer-trimer coupling not shown
3. 2007 Medium

   Extended SNX2 function to a defined physiological/toxin cargo and quantified its redundancy with SNX1 in retrograde transport.

   Evidence Single and combined siRNA knockdown of SNX1/SNX2 with Shiga toxin trafficking assay in Vero cells

   PMID:17498660

   Open questions at the time

   • Single lab, single method
   • Does not distinguish direct cargo engagement from general retromer disruption
   • Residual ~20% transport after double knockdown unexplained
4. 2013 Medium

   Showed that an SNX2-ABL1 gene fusion creates a constitutively active kinase, framing SNX2 as a fusion partner in a leukemogenic oncoprotein with altered TKI sensitivity.

   Evidence Retroviral expression of SNX2-ABL1 in Ba/F3 cells, IL-3 independence and imatinib/dasatinib sensitivity assays

   PMID:24367893

   Open questions at the time

   • Contribution of the SNX2 portion to localization or signaling not dissected
   • Single lab, model cell line only
   • No patient-level functional validation described
5. 2022 Medium

   Revealed a non-canonical SNX2 role beyond retrograde sorting: nucleating endosome-ER contact sites that support autophagy under nutrient stress.

   Evidence Live-cell imaging, siRNA knockdown and co-localization in starved mammalian cells; VAPB interaction

   PMID:36585258

   Open questions at the time

   • Limited biochemical validation of the SNX2-VAPB interaction
   • Direct vs indirect binding to VAPB not resolved
   • Mechanistic link between tubulation and autophagosome formation incomplete
6. 2024 Medium

   Reconstituted SNX2 (in ESCPE-1) membrane-deforming activity with purified proteins and showed VARP, not ESCPE-1 alone, couples ESCPE-1 to Retromer in a proposed endosomal supercomplex.

   Evidence In vitro liposome tubulation with purified mammalian proteins and AlphaFold2-Multimer modeling (preprint)

   PMID:bio_10.1101_2024.07.11.603126

   Open questions at the time

   • Preprint, not peer reviewed; single study
   • Supercomplex existence in cells not demonstrated
   • Stoichiometry and regulation of VARP-mediated assembly unresolved

• How SNX2 cargo selectivity is partitioned from SNX1, and how its retrograde, contact-site, and supercomplex functions are coordinated in cells, remains unresolved.
• No cargo specificity map distinguishing SNX2 from SNX1
• In-cell evidence for the SNX27/ESCPE-1/Retromer supercomplex lacking
• Regulation switching SNX2 between retrieval and ER-contact roles unknown