Structure, Function and Regulation of Fps1p. A Eukaryotic Solute Efflux Channel
Integral membrane proteins belonging to the Major Intrinsic Protein (MIP) family can be found in all kingdoms of life. MIPs are transporters of water, small neutral solutes and possibly ions, and hence are important for osmoregulation at different levels. The aim of this thesis has been to study the structure-function relationship of an atypical member of this family, Fps1p, which is a glycerol facilitator from the yeast Saccharomyces cerevisiae. Fps1p is located in the plasma membrane and mediates efflux of the compatible solute glycerol in the cell's adaptation to lower external osmolarity. Fps1p does not contain the two highly conserved NPA motifs in its channel forming loops, which might have consequences for the transport function and/or the specificity of this MIP. Furthermore, Fps1p is exceptionally large compared to other MIPs due to long hydrophilic extensions in both termini, and these domains have been shown to be crucial for the regulation of glycerol transport.
Fps1p can be characterised in vivo in whole yeast cells, since its function is reflected in cell growth under different osmotic conditions. A short domain, twelve amino acids from the transmembrane domain in each terminus, has been found to be important for proper regulation. Moreover, the N-terminus seems to act as a restriction domain, since higher activity is observed when this domain is deleted. The mechanism is not clear, but Fps1p is probably gated by conformational changes upon shifts in external osmolarity. The Fps1p membrane core has been modelled based on the known structure of the Escherichia coli glycerol facilitator, GlpF, which is its close homologue. Much effort has been invested in production of pure Fps1p for functional and structural studies. Fps1p has been overexpressed in several systems using the hosts S. cerevisiae, Pichia pastoris and E. coli.
In this thesis Fps1p has been compared with MIP proteins to be able to understand more about the structure, transport and the specificity as well as with other solute efflux channels in order to understand more about its regulation.