BET1

BET1 is a SNARE protein that mediates the fusion of ER-derived vesicles with the ERGIC and cis-Golgi during ER-to-Golgi transport (PMID:2192256, PMID:34779586). It was first defined genetically in yeast, where it acts in an essential ER-to-Golgi pathway together with BOS1 and SEC22, with which it interacts non-redundantly (PMID:2192256). In mammalian cells, BET1 assembles into a SNARE complex with syntaxin 5, Ykt6, and GS28 (GOSR1) to drive a late stage of ER-to-Golgi transport (PMID:11323436), and functions with GOSR2, SEC22b, and syntaxin 5 in vesicle fusion, additionally contacting ERGIC-53 at the ERGIC compartment (PMID:34779586). Loss-of-function BET1 variants impair ER-to-Golgi transport and cause mislocalization of ERGIC-53, defining BET1 as the basis of a human disorder of ER-to-Golgi trafficking (PMID:34779586). Beyond this canonical role, in invasive cancer cells MT1-MMP recruits BET1 to MT1-MMP-positive endosomes, where it forms a non-canonical SNARE complex with syntaxin 4 to promote MT1-MMP trafficking to invadopodia (PMID:31519727).

1. 1990 High

   Establishing that BET1 is a dedicated component of the ER-to-Golgi secretory pathway and acts in concert with specific partners answered where in trafficking this gene functions.

   Evidence Genetic suppression and double-mutant lethality analysis in S. cerevisiae

   PMID:2192256

   Open questions at the time

   • Genetic epistasis does not establish physical complex membership or molecular activity
   • Does not define the mammalian orthologs or partners
   • No direct demonstration of vesicle fusion
2. 2001 High

   Demonstrating that mammalian Bet1 physically assembles into a defined SNARE complex resolved its molecular mechanism as a fusion-mediating SNARE acting at a late ER-to-Golgi step.

   Evidence Co-immunoprecipitation and in vitro ER-Golgi transport assay with antibody and recombinant protein inhibition

   PMID:11323436

   Open questions at the time

   • Stoichiometry and structural arrangement of the complex not resolved
   • Precise membrane (vesicle vs target) on which Bet1 resides not assigned
   • Regulation of complex assembly not addressed
3. 2019 Medium

   Showing that MT1-MMP can divert Bet1 into a non-canonical endosomal SNARE complex established that BET1 has a context-dependent role beyond constitutive ER-to-Golgi transport.

   Evidence Fluorescence co-localization, co-IP of SNARE components, and loss-of-function trafficking readout in invasive cancer cells (single lab)

   PMID:31519727

   Open questions at the time

   • Single-lab finding without independent replication
   • Mechanism of MT1-MMP-driven recruitment of Bet1 unknown
   • Generality across cancer types and physiological contexts unclear
4. 2021 High

   Identifying loss-of-function BET1 variants in patients and the BET1/ERGIC-53 interaction connected the transport function to a human disease mechanism.

   Evidence Endogenous co-IP, in silico modeling, and patient-derived fibroblast transport and localization assays across two families

   PMID:34779586

   Open questions at the time

   • Structural basis of the BET1/ERGIC-53 interaction not experimentally resolved
   • Full spectrum of clinical phenotype and genotype-phenotype correlation not defined
   • How variant-induced ERGIC-53 mislocalization produces pathology mechanistically unclear

• How BET1's canonical ER-to-Golgi function is balanced against its recruitment into alternative endosomal complexes, and the structural determinants of its complex assembly, remain open.
• No structural model of BET1-containing SNARE complexes in the corpus
• Switch governing canonical vs MT1-MMP-associated function uncharacterized
• Regulatory inputs controlling BET1 partner choice unknown