Syntaxin 2 splice variants exhibit differential expression patterns, biochemical properties and subcellular localizations

The syntaxins are a large protein family implicated in the targeting and fu sion of intracellular transport vesicles. A subset of proteins of this fami ly are the four syntaxin 2 splice variants, syntaxins 2A (2), 2B (2'), 2C ( 2") and 2D, Each syntaxin 2 variant contains an identical, or nearly identi cal, amino-terminal cytoplasmic domain followed by a distinct hydrophobic ( syntaxins 2A and 2B) or hydrophilic (syntaxins 2C and 2D) carboxyl-terminal domain. To investigate whether the difference among the syntaxin 2 variant s is functionally important, we have examined comparatively their RNA trans cript and protein expression patterns, membrane associations, protein-prote in interactions and intracellular localizations. Analysis of the RNA transc ript and protein expression patterns demonstrated that syntaxins 2A, 2B and 2C are broadly, but not uniformly, expressed while syntaxin 2D expression is restricted to the brain. Subcellular fractionation studies showed that s yntaxins 2A and 2B behave as integral membrane proteins while syntaxin 2C i s only partially associated with membranes. In vitro biochemical assays dem onstrated that the syntaxin 2 variants exhibit similar yet distinct interac tions with other proteins implicated in vesicular trafficking, including SN AP-25, SNAP-23, VAMP-2 and n-sec1, In a variety of nonpolarized cell types, syntaxins 2A and 2B localized to both the plasma membrane and endosomal me mbranes. However, in two polarized epithelial cell lines, MDCK and Caco-2, syntaxin 2A localized predominantly to the apical plasma membrane while syn taxin 2B was associated with both the apical and the basolateral membranes. These observations indicate that the distinct carboxyl-terminal domains of the syntaxin 2 variants influence their biochemical and localization prope rties and may therefore confer upon these variants different functional rol es in the regulation of intracellular membrane trafficking.