Cd44

Protein

ACHE, acetylcholinesterase; AQPI, aquaporin 1, water transporter; FUT, fucosyltransferase; DAF, complement regulatory molecule-decay accelerating factor; CRI, C3b/C4b receptors.

ACHE, acetylcholinesterase; AQPI, aquaporin 1, water transporter; FUT, fucosyltransferase; DAF, complement regulatory molecule-decay accelerating factor; CRI, C3b/C4b receptors.

tensive patients. Ca2 f fluxes in the red cells are regulated by another transport system constituted by an enzyme stimulated by calmodulin which improves its affinity for Ca2+. Increase of Ca2+ sharply impairs the deformability of the erythrocyte. The Ca2' pump activity appears to be influenced by thyroid hormone.

Band 3 is the conduit for the important chloride-bicarbonate exchange, but it is also the major binding site for a variety of enzymes. The 40 kDa N-terminal cytoplasmic domain binds and inhibits the glycolytic enzymes glucose-3-phosphate dehydrogenase (G3PDH), phosphoglycerate kinase (PGK), aldolase, and hemoglobin too, and modulates the binding of band 3 to protein 4.1, pallidin and ankyrin. The 52 kDa transmembrane domain is composed of 12 or 14 transmembrane helices which form the transport channel. The 1.2 million channels in each red cell can exchange 1010-10n HCO? and CI anions s "1. An anion moves by a 'ping-pong' mechanism in which an intracellular anion enters the transport channel and is translocated outward and released (outward conformation) until the penetration of the channel by an extracellular anion triggers the reverse cycle. The mechanism involves fairly large conformational changes.

Receptors

The red cell membrane is provided with several receptor activities. Some of them include receptors for hormones like insulin (not utilized for glucose transport) or thyroid hormones. In addition (3-adrenergic receptors (possibly playing a role in the transport of Mg2+) and acetylcholine receptors have been demonstrated. Erythrocytes bear a complement receptor (CR1) which is able to bind C4b molecules attached to the red cell membrane during com plement activation. No transferrin receptors have been detected on the mature erythrocytes, these structures being lost at the reticulocyte stage. Putative receptors for prostaglandin E, parathyroid hormone, diphtheria toxin, ceruloplasmin and opiates have been reported.

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