Major intrinsic proteins are a large family of transmembrane protein channels that are grouped together on the basis of sequence similarities. Proteins from this family exhibit essentially two distinct types of channel properties: (1) specific water
transport by the aquaporins, and (2) small neutral solutes
transport, such as glycerol by the glycerol facilitators.
MIP family includes the following channels:
- Mammalian major intrinsic protein (MIP). MIP is the major component of lens fibre gap junctions.
- Mammalian aquaporins. These proteins form water- specific channels that provide the plasma membranes of red cells and kidney prox imal 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 : alpha (seed), gamma, 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 fac ilitator (gene pduF).
- Yeast FPS1, a glycerol uptake/efflux facilitator protein.
- Drosophila neurogenic protein 'big brain' (bib). This protein may mediate in tercellular communication; it may functions by allowing the transport of certain molecules(s) and thereby sending a signal for an exodermal cell to become an ep idermoblast instead of a neuroblast.
- Yeast hypothetical protein YFL054c.
- A hypothetical protein from the pepX region of Lactococcus lactis.
MIP family proteins are thought to contain 6 TM domains. Sequence analysis suggests that the proteins may have arisen through tandem, intragenic duplication from an ancestral protein that contained 3 TM domains.
Some of the proteins in this group are responsible for the molecular basis of the blood group antigens, surface markers on the outside of the red blood cell membrane. Most of these markers are proteins, but some are carbohydrates attached to lipids or proteins. Aquaporin-CHIP (Aquaporin 1) belongs to the Colton blood group system and is associated with Co(a/b) antigen.
Human proteins containing this domain