Received September 16, 2005
Revised September 19, 2005
Accepted after revision October 6, 2005
GI and epithelial physiology
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Physiology of Anion Transport Symposium Paper: CFTR, chloride concentration and cell volume - could mammalian protein histidine phosphorylation play a latent role?
Kate J Treharne 1,
Russell M Crawford 1,
Anil Mehta 1*
1 University of Dundee
* To whom correspondence should be addressed. E-mail: a.mehta{at}dundee.ac.uk.
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Abstract |
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A considerable body of evidence indicates that the intracellular chloride concentration ([Cl-]i) is an important regulatory signal in epithelial ion transport. [Cl-]i regulates the open channel probability of sodium and chloride channels, the rate of chloride channel recycling to the apical membrane, cell volume homeostasis, the activity of sodium-coupled chloride entry pathways and G-protein activity. Cell volume goes awry in epithelial cells bearing mutant forms of the cystic fibrosis (CF) transmembrane conductance regulator protein (CFTR), however, the pathways that mediate this [Cl-]i effect at the apical membrane of polarised epithelia are unknown. Recently, we proposed a mechanism for the transduction of in vitro chloride concentration into a phosphorylation signal to proteins within the apical membrane of respiratory epithelia. Our studies show that an apically-enriched plasma membrane fraction from a variety of species including sheep, human and mouse airway contains at least two membrane-bound protein kinases which exhibit a number of novel properties. Firstly, the phosphate is located on histidine residues within different families of proteins; one kinase(s) utilises GTP rather than ATP as a phosphate donor and each kinase has its own unique profile of membrane protein phosphorylation (which itself varies with anion species). Secondly, both kinases mediate Cl--dependent phosphorylation of an apical membrane protein around the established physiological values for [Cl-]i in airway epithelial cells (~40mM); associated phosphatases also alter the net phosphoprotein profile of the apical membrane. These findings are reviewed and their potential roles explored in relation to the pathogenesis of CF using the control of cell volume as a paradigm for disrupted cellular function in CF-affected epithelia.
Key Words:
Cell volume, Cystic fibrosis, Phosphorylation
