Orphan receptors function

Orphan receptor, a gene product that is predicted to be a receptor through structure and spontaneous interaction with G-proteins but for which there is no known endogenous ligand or physiological function. 

One-way analysis of variance, 229-230, 230f—231f Operational model derivation of, 54-55 description of, 45—47, 46f function for variable slope, 55 for inverse agonists, 221 of agonism, 47f orthosteric antagonism, 222 partial agonists with, 124, 220-221 Opium, 147 Orphan receptors, 180 Orthosteric antagonism... 

In addition to the receptors mentioned in A, the family of steroid receptors also includes the product of the oncogene erb-A (see p. 398), the receptor for the environmental toxin dioxin, and other proteins for which a distinct hormone ligand has not been identified (known as orphan receptors ). Several steroid receptors—e. g., the retinoic acid receptor-form functional heterodimers with orphan receptors. 

There are also orphan receptors known which bind as monomers to asynnnetric recognition sequences (fig. 4.7d). The function of these receptors is still poorly characterized. 

The diversity of nuclear receptor systems is also demonstrated by the fact that orphan receptors exist, which exercise a constitutive repressive or activating function independent of ligand. 

The availability of large amounts of raw sequence data, due to public EST collections, and the ability of scientists to access this data, has led to a massive increase in the number of novel proteins identified in silico—that is purely on computational techniques. Nowhere has this been as fruitful as in the chemokine area where there has been an explosive increase in the number of chemokines identified. The challenge now is to piece together the roles that these new chemokines play in routine immunosurveillance and also in inflammatory diseases. This challenge has been taken up in the last year with the identification of ligands for four chemokine receptors previously classified as orphans. In terms of receptor function, we still have a long way to go, since there are still many chemokines for which receptors have not been identified, but at least we have a more complete list of the players as far as the ligands are concerned. 

The answer to this question is not known. However, homology-based molecular cloning techniques have led to the identification of literally hundreds of these receptors each of which conforms to the seven transmembrane domain pattern. The function of many of these receptors and their ligands are unknown such receptors are referred to as orphan receptors. Similarly, there is a considerable diversity of G-proteins. In particular, over 30 different a-subunit types have been described. 

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