Minisymposium 17: Water

Abs # 32002: Analysis of the putative pore-selectivity regions of Arabidopsis Major Intrinsic Proteins by homology modeling

Presenter:	Wallace, Ian S, iwallace@utk.edu
Authors	Wallace, Ian S (A)   Roberts, Daniel M (A)
Affiliations:	(A): The University of Tennessee, Knoxville Department of Biochemistry Cellular and Molecular Biology

Major Intrinsic Proteins (MIPs) are a family of membrane channels that facilitate the bidirectional transport of water and small uncharged solutes such as glycerol. Using computational methods we have constructed homology models of the putative pore regions for the 35 full-length MIPs of Arabidopsis thaliana using the X-ray crystal structures of mammalian aquaporin1 and the bacterial glycerol permease, GlpF as templates. Models of the Arabidopsis MIPs show excellent structural agreement with one another and with the experimental structures, possessing all of the hallmarks of MIPs, including six membrane-spanning alpha helices and two NPA half-helices. Based on comparisons of the narrow selectivity filter regions of the pores (the aromatic/arginine filter or ar/R region), the members of the four phylogenetic subfamilies of Arabidopsis MIPs can be classified into eight subgroups. PIPs (Plasma membrane Intrinsic Proteins) possess a uniform ar/R signature characteristic of high water transport aquaporins, whereas the TIP (Tonoplast Intrinsic membrane protein) subfamily is highly diverse with three separate conserved ar/R regions, two of which are largely divergent from the canonical ar/R region of animal aquaporins. NIPs (Nodulin 26-like Intrinsic Proteins) possess two separate conserved ar/R regions, one which is similar to the archetype, soybean (Glycine max) nodulin 26, and another which is characteristic of Arabidopsis NIP6;1. The analysis of the NIP ar/R region by site-directed mutagenesis, modeling, and solute transport suggests a wider aperture and greater hydrophobicity which confers unique solute transport properties and selectivity. The SIP (Small basic Intrinsic Protein) subfamily possesses two ar/R subgroups, characteristic of either SIP1 or SIP2. Both SIP ar/R residues are divergent from all other MIPs in plants and other kingdoms. Overall, these findings suggest that higher plants MIPs have a common fold but show distinct differences in pore apertures, potential to form hydrogen bonds with transported molecules, and amphiphilicity that result in divergent transport selectivities. (Supported by an ASPB SURF award to ISW and by NSF grant MCB-0237219)