|How the Membrane Protein AmtB Transports Ammonia| Membrane
proteins provide molecular-sized entry and exit portals for the
various substances that pass into and out of cells. While life
scientists have solved the structures of protein channels for
ions, uncharged solutes, and even water, up to now they have
only been able to guess at the precise mechanisms by which gases
(such as NH3, CO2, O2, NO, N2O, etc.) cross biological
membranes. But, with the first high-resolution structure of a
bacterial ammonia transporter (AmtB), determined by a team in
the Stroud group from the University of California, San
Francisco, it is now known that this family of transporters
conducts ammonia by stripping off the proton from the ammonium
(NH4+) cation and conducting the uncharged NH3 “gas.” Progress
in determining structures of membrane proteins of all kinds has
been slowed by the difficulty of obtaining sufficiently robust
crystals that diffract to high resolution. A common strategy is
to grow crystals of proteins from mul