Aquaporin

Aquaporin, aka the "plumbing system for cells", is a transport protein that mediates the passage of water molecules in and out of a cell. It is a tetramer (has four identical units) with each subunit creating a channel. This protein is very important for our kidney which processing a large amount of water on a daily basis.
Peter Agre who discovered this protein was awarded the Nobel Prize in Chemistry in 2003.