Syntaxin 1A inhibits regulated CFTR trafficking in Xenopus oocytes

The cystic fibrosis transmembrane conductance regulator (CFTR) is an epithe lial cell Cl channel, whose gating activity and membrane trafficking are co ntrolled by cAMP/protein kinase A (PKA)mediated phosphorylation. CFTR Cl cu rrents are regulated also by syntaxin lb (A. P. Naren, D. J. Nelson, W. W. Xie, B. Jovov, J. Pevsner, M. K. Bennett, D. J. Benos, M. W. Quick, and K. L. Kirk. Nature 390: 302-305, 1997), a protein best known for its role in m embrane trafficking and neurosecretion. To examine the mechanism of syntaxi n 1A inhibition, we expressed these proteins in Xenopus oocytes and monitor ed agonist-induced changes in plasma membrane capacitance and cell surface fluorescence of CFTR that contains an external epitope tag, cAMP stimulatio n elicited large increases in membrane capacitance and in cell surface labe ling of flag-tagged CFTR. Coexpression of CFTR with syntaxin 1A, but not sy ntaxin 3, inhibited cAMP-induced increases in membrane capacitance and plas ma membrane CFTR content. Injection of botulinum toxin/C1 rapidly reversed syntaxin's effects on current and capacitance, indicating that they cannot be explained by an effect on CFTR synthesis. Functional expression of other integral membrane proteins, including Na-coupled glucose transporter hSGLT 1, inwardly rectified K channel hIK1, P2Y2 nucleotide receptor, and viral h emagglutinin protein, was not affected by syntaxin 1A coexpression. These f indings indicate that acute regulation of the number of CFTR Cl channels in plasma membrane is one mechanism by which cAMP/PKA regulates Cl currents. Inhibition of plasma membrane CFTR content by syntaxin 1A is consistent wit h the concept that syntaxin and other components of the SNARE machinery are involved in regulated trafficking of CFTR.