The structure of aquaporin -1 has been solved. it is a tetramer. But more like
chloride channel, each of its four subunit has an independant pore. So the
tetramer actually is composed by four channels. (see figure below)
The selection problem is solved by arranging three hydrophlic patches along
the
hydrophobic channel wall. This hydrophilic patches contribute four water
binding sites and form the selective filter. Look at the following figure.
Four water molecules (green sphere) are shown here,
