According to a press release (18 December 2001), a team of researchers led biophysicist Bing Jap led a team from Lawrence Berkeley National Laboratory's Life Sciences Division have determined the structure and function of a cell membrane protein, called aquaporin 1 (AQP1), that is specific for water molecules. The structure reveals the how the AQP1 can transport water through the cell membrane at a high rate while effectively blocking everything else that is larger or smaller, even individual protons, the nuclei of hydrogen atoms.
Each AQP1 channel is made up of four identical subunits, each with an entrance chamber on the outside of the cell envelope, connected to a similar chambeer inside the cell by a long, narrow pore. "The secret of AQP1's specificity is two-fold: it selects for size and for chemical nature," Jap says. "There is a very narrow constriction in the pore, which admits no molecule bigger than water. To keep out molecules smaller than water there is also a chemical filter, formed by the specific orientation and distribution of the amino acid residues lining the pore."
Molecules attempting to enter the channel are bound to water molecules that are stripped away in the pore; charged species are therefore left with net electrical charge. "The filter strongly rejects charged molecules or ions, even as small as single protons," Jap explains. The unique distribution of amino acid residues along the pore wall also accounts for the channel's ability to move water quickly. The channelís internal environment has both hydrophilic and hydrophobic components. Water molecules readily get in because of the hydrophilic sites, but the hydrophobic regions prevent them from binding too frequently. Thus water and only water flows freely in and out of the cell through AQP1's pores, the direction of flow depending only on changing relative pressure inside and outside the cell.
Similar work on the structure and workings of an ion-channel protein sorter for potassium ions was reported here on 2 November 2001.