PROSITE documentation PDOC00193

MIP family signature

Recently the sequence of a number of different proteins, that all seem to be transmembrane channel proteins, has been found to be highly related [1,2,3,4]. These proteins are listed below.

• Mammalian major intrinsic protein (MIP). MIP is the major component of lens fiber gap junctions. Gap junctions mediate direct exchange of ions and small molecule from one cell to another.
• Mammalian aquaporins [5]. These proteins form water-specific channels that provide the plasma membranes of red cells and kidney proximal and collecting tubules with high permeability to water, thereby permitting water to move in the direction of an osmotic gradient.
• Soybean nodulin-26, a major component of the peribacteroid membrane induced during nodulation in legume roots after Rhizobium infection.
• Plants tonoplast intrinsic proteins (TIP). There are various isoforms of TIP: α (seed), γ, Rt (root), and Wsi (water-stress induced). These proteins may allow the diffusion of water, amino acids and/or peptides from the tonoplast interior to the cytoplasm.
• Bacterial glycerol facilitator protein (gene glpF), which facilitates the movement of glycerol across the cytoplasmic membrane.
• Salmonella typhimurium propanediol diffusion facilitator (gene pduF).
• Yeast FPS1, a glycerol uptake/efflux facilitator protein.
• Drosophila neurogenic protein 'big brain' (bib). This protein may mediate intercellular communication; it may functions by allowing the transport of certain molecules(s) and thereby sending a signal for an exodermal cell to become an epidermoblast instead of a neuroblast.
• Yeast hypothetical protein YFL054c.
• A hypothetical protein from the pepX region of lactococcus lactis.

The MIP family proteins seem to contain six transmembrane segments. Computer analysis shows that these protein probably arose by a tandem, intragenic duplication event from an ancestral protein that contained three transmembrane segments. As a signature pattern we selected a well conserved region which is located in a probable cytoplasmic loop between the second and third transmembrane regions.