A Sec1-related vesicle-transport protein that is expressed predominantly in epithelial cells

Sec1‐related proteins are involved in docking and fusion of transport vesicles in eukaryotic cells. Here we report the cloning and molecular characterization of a Sec1‐related protein expressed in the MDCK epithelial cell line. This protein represents a canine counterpart of the murine Munc‐18–2/Munc‐18b/muSec1 protein, displays 93% amino acid identity with these proteins, has a similar tissue mRNA expression pattern, and associates in vitro with syntaxins 1A, 2, and 3. In situ hybridization analysis of embryonic mouse tissues revealed prominent expression of the munc‐18‐2 mRNA in the epithelia of several tissues. Cell‐fractionation studies demonstrated that the majority of Munc‐18‐2 is membrane associated. Most of the protein is washed off the membranes by sodium carbonate, pH 11.5. However, the protein is poorly solubilized by detergent treatment. The Munc‐18‐2 protein was localized, by immunofluorescence microscopy, to the plasma membrane of MDCK cells, and is apically distributed in the epithelial cells of mouse tissues. When overexpressed in COS‐1 cells, the protein appeared to be largely cytosolic. However, upon expression with syntaxin 1A, it displayed a shift to the plasma membrane, where the two proteins colocalized. These results identified Munc‐18‐2 as a predominantly epithelial vesicle‐transport protein with a polarized distribution and provided novel in vivo evidence for the association of Sec1‐related proteins with members of the syntaxin family.