Plant major intrinsic proteins. Identification and characterization of novel water channel-like protiens.

The Major Intrinsic Proteins, MIPs, constitute a large, ancient superfamily of membrane proteins that are found in all kingdoms of life. Most MIPs facilitate passive diffusion of water across biological membranes (aquaporins), although many channels are also permeable for glycerol and other small uncharged solutes, (aquaglyceroporins). Other substrates reported for specific MIPs are anions, carbon dioxide, arsenic and boric acid. MIPs are predominantly important in processes involving water regulation and osmoregulation, but have also been suggested to be involved in carbohydrate metabolism.

In the genome of the model plant Arabidopsis thaliana, 35 MIP-encoding genes were identified. By sequence homology and phylogenetic analyses, these genes were divided into four subfamilies, Plasma membrane Intrinsic Proteins, PIPs, Tonoplast Intrinsic Proteins, TIPs, Nod26-like Intrinsic Proteins, NIPs, and Small basic Intrinsic proteins, SIPs. Based on the phylogenetic analyses and the already commonly used names, a new nomenclature, which now is widely accepted, was proposed.

The SIPs form the most recently identified subfamily, and this divergent family is also the poorest characterized. The constriction region of SIPs is unlike filters of both aguaglyceroporins and aquaporins, suggesting different substrates than glycerol or water. Expression studies by promoter::GUS fusions imply that SIP1;1 is expressed in roots, trichomes, stigma, style and stamen. Vascular root localization of SIP1;1 was also observed on the protein level by immunogold labeling, which also labeled guard cells in leaves. SIP2;1 was also localized in guard cells in addition to roots, predominantly in the center of the vasculature. The subcellular localization was investigated by aqueous two-phase partitioning and immunoblotting. Both SIP1;1 and SIP2;1 appear in the plasma membrane fraction when isolated from leaves, but mainly in the fraction containing internal membranes when isolated from roots. Water and glycerol permeability for SIP1;1 and SIP2;1 was investigated by heterologous expression in Xenopus laevis oocytes, but no permeability for either molecule could be observed.

In the moss Physcomitrella patens a MIP more similar to bacterial aquaglyceroporins than to any other known plant MIP was identified. Most likely, this protein, GIP1;1, was recruited by plants from bacteria by horizontal gene transfer. GIP1;1 (GIP for GlpF-like Intrinsic Protein) was predicted to be a glycerol selective channel, which subsequently was confirmed after heterologous expression in Xenopus oocytes.