A new subfamily LIP of the major intrinsic proteins

Background: Proteins of the major intrinsic protein (MIP) family, or aquaporins, have been detected in almost all organisms. These proteins are important in cells and organisms because they allow for passive transmembrane transport of water and other small, uncharged polar molecules. Results: We compared the predicted amino acid sequences of 20 MIPs from several algae species of the phylum Heterokontophyta (Kingdom Chromista) with the sequences of MIPs from other organisms. Multiple sequence alignments revealed motifs that were homologous to functionally important NPA motifs and the so-called ar/R-selective filter of glyceroporins and aquaporins. The MIP sequences of the studied chromists fell into several clusters that belonged to different groups of MIPs from a wide variety of organisms from different Kingdoms. Two of these proteins belong to Plasma membrane intrinsic proteins (PIPs), four of them belong to GlpF-like intrinsic proteins (GIPs), and one of them belongs to a specific MIPE subfamily from green algae. Three proteins belong to the unclassified MIPs, two of which are of bacterial origin. Eight of the studied MIPs contain an NPM-motif in place of the second conserved NPA-motif typical of the majority of MIPs. The MIPs of heterokonts within all detected clusters can differ from other MIPs in the same cluster regarding the structure of the ar/R-selective filter and other generally conserved motifs. Conclusion: We proposed placing nine MIPs from heterokonts into a new group, which we have named the LIPs (large intrinsic proteins). The possible substrate specificities of the studied MIPs are discussed.

背景：主要内在蛋白（major intrinsic protein, MIP）家族蛋白，又称水通道蛋白（aquaporins），几乎在所有生物中均已被检出。这类蛋白在细胞及生物体内发挥关键作用，可介导水与其他小型不带电荷极性分子的被动跨膜转运。结果：我们将不等鞭毛门（Heterokontophyta，色藻界（Chromista））多个藻类物种的20种MIP的预测氨基酸序列，与其他生物的MIP序列进行了比对分析。多序列比对结果显示，存在与功能重要的NPA基序以及甘油通道蛋白和水通道蛋白的所谓ar/R选择性过滤器同源的保守基序。本次研究涉及的色藻MIP序列可划分为多个簇，这些簇分别隶属于来自不同界的各类生物的不同MIP类群。其中2种蛋白属于质膜内在蛋白（Plasma membrane intrinsic proteins, PIPs），4种属于GlpF样内在蛋白（GlpF-like intrinsic proteins, GIPs），1种属于绿藻特有的MIPE亚家族。另有3种属于未分类MIP，其中2种源自细菌。本次研究的8种MIP在多数MIP典型具备的第二个保守NPA基序的位置，含有NPM基序。不等鞭毛类的MIP在所有检出的簇中，其ar/R选择性过滤器与其他普遍保守的基序结构，可能与同一簇内的其他MIP存在差异。结论：我们提议将来自不等鞭毛类的9种MIP划分为一个新的类群，并将其命名为LIPs（大型内在蛋白，large intrinsic proteins）。本文还讨论了本次研究的MIP可能具备的底物特异性。