Polymorphism in the packing of aquaporin-1 tetramers in 2-D crystals

Hitherto, the packing arrangement of the aquaporin-1 (AQP1) tetramer in a-d imensional (2-D) crystals (two-sided plane group p42(1)2) was observed to b e largely similar (canonical crystal form) despite the difference in the so urce of the protein, the glycosylation state of the protein, the type of li pids, and the ratio of lipid to protein in the crystallization mixture. We report here our observation that the packing of AQP1 tetramers shows polymo rphism in 2-D crystals generated in dioleoyl phosphatidylcholine bilayers. Apart from the canonical form, three additional allomorphs were identified. One was observed when small. (0.25) lipid to protein ratio was used in the crystallization mixture while the other two were observed when the divalen t cation content in the canonical crystals was modified. The various allomo rphs were distinguished by different relative orientations of the AQP1 tetr amer viewed in projection. The same, two-sided plane group p4212 and simila r unit cell dimensions were maintained in the different allomorphs as estab lished by analysis of images of frozen-hydrated, nominally untilted crystal s. Our results indicate that the interaction between the AQP1 monomers at t he interface of the tetramers is flexible and is also strongly influenced b y Mg2+ ions with the cation effect materializing because of the intrinsic f luidity of the membrane. (C) 2000 Academic Press.