Major Intrinsic Proteins - Structure, function, interactions

Sammanfattning: Major intrinsic proteins, also referred to as aquaporins (AQPs) are localized in membranes of all domains of life to increase the permeability to water or other small, polar molecules. Several monophyletic groups can be discerned in this large and divergent protein superfamily. Members within these subfamilies commonly show a similar substrate selectivity established by conserved filters. Comparison of structurally known selectivity filters have elucidated some of the mechanisms underlying substrate profiles. However, the physiological function of these diverse and abundant proteins can only be fully appreciated in combination with an understanding of their regulatory properties. This work provides the first molecular view on a tonoplastic protein, AtTIP2;1, which is situated in the membrane of the plant vacuole and permeable to water, ammonia, but not glycerol. The novel selectivity filter with a fifth residue in direct contact to substrates is compared to structures of other subfamilies to explain the present selectivity. The mechanism of permeation is investigated with mutations, stopped-flow and by growth assays. The animal AQP8s, which have similar substrate profiles, are modeled on the plant protein and their expected functions are discussed in the light of their structures. Furthermore, protein interactions and interactions with small molecules are analyzed to develop a better understanding of potential regulatory features.