Ammonium transport proteins widely exist in various life forms including bacteria, fungi, plants, animals, etc. The transport of NH₃/NH₄⁺ through an ammonium transport protein has been widely studied. However, the issue that the ionic NH₄⁺ or the electrically neutral NH₃ species truly goes through the hydrophobic pore of an ammonium transport protein remains controversial. This review surveys the progress in studying on the transport mechanisms of NH₃/NH₄⁺ through several typical ammonium transport proteins. The main mechanisms include the single transport of NH₃ or NH₄⁺, and the cooperative transport of NH₃ and H⁺, etc.

Contents
1 Introduction
2 NH₃ transport mechanism
3 NH₄⁺ transport mechanism
4 Cooperative transport of NH₃ and H⁺
5 Other transport mechanisms
6 Conclusion