Understanding microbial MIP channels

The microbial members of the MIP family of transmembrane channel proteins are attracting considerable interest, and a topical review of their structural and biological features was provided recently by Hohmann et al.1 MIP proteins are widely distributed throughout nature. Already, 220 MIP family members, including 74 of microbial origin, have been identified and partially characterized; many more will be identified as a result of genome sequencing. In spite of their 2.5–3 billion years of evolutionary history, microbial MIP proteins are structurally similar to their invertebrate and vertebrate counterparts2. However, unlike MIP proteins from higher organisms, the linear sequences of which permit us to distinguish homologues highly selective for water (aquaporins, AQP) and homologues permeable to glycerol and other small neutral solutes in addition to water (aquaglyceroporins3), the correlation between sequence and functional properties of microbial MIPs has not yet been fully assessed.