1 Department of Biology, Portland State University, Portland, OR 97207, USA, Department of Membrane Transport Biophysics, Institute of Physiology, Academy of Sciences of the Czech Republic, Videnska 1083, 14220, Prague, Czech Republic.
The publisher regrets that this article is an accidental duplication of an article that has already been published in Biochim. Biophys. Acta, 1659 (2004) 83-91, doi:10.1016/j.bbabio.2004.03.019. The duplicate article has therefore been withdrawn.