UNC-18 promotes both the anterograde trafficking and synaptic function of syntaxin.
The SM protein UNC-18 has been proposed to regulate several aspects of secretion, including synaptic vesicle docking, priming, and fusion. Here, we show that UNC-18 has a chaperone function in neurons, promoting anterograde transport of the plasma membrane soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) protein Syntaxin-1. In unc-18 mutants, UNC-64 (Caenorhabditis elegans Syntaxin-1) accumulates in neuronal cell bodies. Colocalization studies and analysis of carbohydrate modifications both suggest that this accumulation occurs in the endoplasmic reticulum. This trafficking defect is specific for UNC-64 Syntaxin-1, because 14 other SNARE proteins and two active zone markers were unaffected. UNC-18 binds to Syntaxin through at least two mechanisms: binding to closed Syntaxin, or to the N terminus of Syntaxin. It is unclear which of these binding modes mediates UNC-18 function in neurons. The chaperone function of UNC-18 was eliminated in double mutants predicted to disrupt both modes of Syntaxin binding, but it was unaffected in single mutants. By contrast, mutations predicted to disrupt UNC-18 binding to the N terminus of Syntaxin caused significant defects in locomotion behavior and responsiveness to cholinesterase inhibitors. Collectively, these results demonstrate the UNC-18 acts as a molecular chaperone for Syntaxin transport in neurons and that the two modes of UNC-18 binding to Syntaxin are involved in different aspects of UNC-18 function.
McEwen
Jason M
JM
Department of Molecular Biology, Massachusetts General Hospital, Harvard Medical School, Boston, MA 02114, USA.
Kaplan
Joshua M
JM
D000818:Animals; D017173:Caenorhabditis elegans; D029742:Caenorhabditis elegans Proteins / Q000378:metabolism*; D004721:Endoplasmic Reticulum / Q000378:metabolism; D005786:Gene Expression Regulation; D006026:Glycoside Hydrolases / Q000378:metabolism; D049452:Green Fluorescent Proteins / Q000378:metabolism; D008954:Models, Biological; D009154:Mutation; D009474:Neurons / Q000378:metabolism; D010750:Phosphoproteins / Q000378:metabolism*; D017434:Protein Structure, Tertiary; D021381:Protein Transport; D050765:Qa-SNARE Proteins / Q000378:metabolism*; D013569:Synapses / Q000378:metabolism*; D033921:Vesicular Transport Proteins / Q000378:metabolism*
D029742:Caenorhabditis elegans Proteins; D010750:Phosphoproteins; D050765:Qa-SNARE Proteins; C082636:Unc-18 protein, C elegans; D033921:Vesicular Transport Proteins; D049452:Green Fluorescent Proteins; D006026:Glycoside Hydrolases