Glyceroporin
Cartoons view of one monomer of glyceroporin. One half is colored cyan and one half is colored violet. The connection is in gray.
The NPA sequences are in spacefilling. Notice the helices that meet in the middle.
A narrow channel is visible. This allows glycerol molecules to travel in single file through the protein.
Fu, D., Libson, A., Miercke, L. J. W., Weitzman, C., Nollert, P., Krucinski, J., Stroud, R. M.: Structure of a Glycerol Conducting Channel and the Basis for its Selectivity Science 290 pp. 481 (2000) (PubMed)
1fx8 (PDB)