Pore selectivity analysis of an aquaglyceroporin by stopped-flow spectrophotometry on bacterial cell suspensions

Background information. Transport of water and small neutral solutes across plasma membranes is facilitated by AQP (aquaporin) and aquaglyceroporin channels, which belong to the MIP (major intrinsic protein) family. So far, more than 800 MIP proteins have been identified on the basis of sequence homology, but only less than 10% of them have been functionally characterized. In most studies, the channel properties of MIP proteins have been determined by using Xenopus oocyte swelling assays or stopped-flow spectrophotometry on proteoliposomes. As both methods sometimes present disadvantages, we developed an alternative method for analysing MIP function.Results. The kinetics of plasmolysis or deplasmolysis of Escherichia coli cells in suspension, in response to osmotic challenges, was analysed by stopped-flow spectrophotometry. Cytoplasmic volume variations were monitored either by GFP (green fluorescent protein) fluorescence quenching or by 90 degrees scattered light. The single exponential response to up-shocks in the impermeant solute mannitol was strongly accelerated when the cells expressed the native E. coli AQP AqpZ (rate constant 37.24 versus 3.05 s(-1) for control cells). The responses to hyperosmotic shocks realized with glycerol were biphasic. First, a light-scattering increase corresponded to cell plasmolysis. Secondly, deplasmolysis occurred when glycerol entered into the cell. Both phases were accelerated when the aquaglyceroporin GlpF was present in cell membranes. We concluded that the behaviour of MIP-expressing bacteria in the stopped-flow system was qualitatively identical with that reported for MIP-expressing oocytes or MIP-containing proteoliposomes. We then used this system to analyse the effects of mutations in the pore constriction of Gla(Llac), the aquaglyceroporin from Lactococcus lactis. In the present study, we show that Gla(Llac) loses its ability to transport glycerol but retains its ability to transport water when Val(223) was replaced by a histidine, the residue at the equivalent position in strict AQPs.Conclusions. These results show that stopped-flow spectrophotometry performed on E. coli cell suspensions is a useful experimental system to analyse the selectivity of wild-type or mutant MIP proteins and that a bifunctional aquaglyceroporin switches to an AQP by a single amino acid mutation in the pore constriction.