Munc18-2, a functional partner of syntaxin 3, controls apical membrane trafficking in epithelial cells.
The Sec1-related proteins bind to syntaxin family t-SNAREs with high affinity, thus controlling the interaction of syntaxins with their cognate SNARE partners. Munc18-2 is a Sec1 homologue enriched in epithelial cells and forms a complex with syntaxin 3, a t-SNARE localized to the apical plasma membrane. We generated here a set of Munc18-2 point mutants with substitutions in conserved amino acid residues. The mutants displayed a spectrum of different syntaxin binding efficiencies. The in vitro and in vivo binding patterns were highly similar, and the association of the Munc18-2 variants with syntaxin 3 correlated well with their ability to displace SNAP-23 from syntaxin 3 complexes when overexpressed in Caco-2 cells. Even the Munc18-2 mutants that do not detectably bind syntaxin 3 were membrane associated in Caco-2 cells, suggesting that the syntaxin interaction is not the sole determinant of Sec1 protein membrane attachment. Overexpression of the wild-type Munc18-2 was shown to inhibit the apical delivery of influenza virus hemagglutinin (HA). Interestingly, mutants unable to bind syntaxin 3 behaved differently in the HA transport assay. While one of the mutants tested had no effect, one inhibited and one enhanced the apical transport of HA. This implies that Munc18-2 function in apical membrane trafficking involves aspects independent of the syntaxin 3 interaction.
Riento
K
K
Department of Biochemistry, National Public Health Institute (KTL), Mannerheimintie 166, FIN-00300 Helsinki, Finland.
Kauppi
M
M
Keranen
S
S
Olkkonen
V M
VM
D000595:Amino Acid Sequence; D001692:Biological Transport; D018938:Caco-2 Cells; D002462:Cell Membrane / Q000378:metabolism / Q000648:ultrastructure; D004847:Epithelial Cells / Q000378:metabolism* / Q000648:ultrastructure*; D006801:Humans; D008565:Membrane Proteins / Q000378:metabolism*; D008969:Molecular Sequence Data; D051938:Munc18 Proteins; D009419:Nerve Tissue Proteins; D011506:Proteins / Q000235:genetics / Q000378:metabolism*; D050765:Qa-SNARE Proteins; D016415:Sequence Alignment; D033921:Vesicular Transport Proteins
D008565:Membrane Proteins; D051938:Munc18 Proteins; D009419:Nerve Tissue Proteins; D011506:Proteins; D050765:Qa-SNARE Proteins; C497940:STXBP2 protein, human; D033921:Vesicular Transport Proteins