It is known that hormones like the glucocorticoid and thyroid hormones enter cells by facilitated diffusion. It is also known that hormones then bind to specific receptor proteins, thereby creating a hormone/receptor complex. The binding of hormone to the receptor initiates an alosteric alteration of the receptor protein. As a result of this alteration, it is believed that the hormone/receptor complex is capable of binding with high affinity to certain specific sites on the chromatin DNA. Such sites, which are referred to as hormone response elements or HRE's, modulate expression of nearby target gene promoters.
A major obstacle to further understanding of the specifics of gene regulation by exogenous inducers such as hormones has been the lack of availability of receptor proteins in sufficient quantity and purity to allow such proteins to be adequately analyzed and characterized. This same lack of availability has thwarted the use of receptors in diagnostic assays to determine the presence of exogenous inducers (e.g., the hormones) in various body fluids and tissues, as well as their use as "prototypes" for engineering chimeric receptor protein analogs.
In an effort to overcome this lack of availability of receptor proteins, scientific investigators are working to discover the genes that encode such proteins. To date several such genes have been disclosed and characterized. The cloned genes include those encoding the following receptors: glucocorticoid, mineralocorticoid, progesterone, estrogen, the two steroid-related receptors (known in the art as ERR1 and ERR2), vitamin D.sub.3, thyroid, v-erb-A, E75 (Drosophilia and two retinoid receptor proteins, retinoic acid receptor alpha (RAR.alpha.) and retinoic acid receptor beta (RAR.beta.). (See Giguere, et al., (1987) regarding RAR.alpha., and Petkovich, et al., (1987) and Brand, et al., (1988) regarding RAR.beta..)
This disclosure describes the isolation and characterization of a cDNA encoding a third functional retinoid receptor protein that is referred to herein as the gamma retinoic acid receptor (RAR.gamma.). Like RARs alpha and beta, the new gamma retinoic acid receptor has homology with the DNA-binding and ligand-binding domains of the steroid and thyroid hormone receptors.
The retinoic acid receptor genes belong to the superfamily of genes known as the steroid hormone receptor family. All genes in this family can be divided into discrete regions or domains that are sometimes referred to as regions A/B, C, D, E, and F. See FIG. 2; also see Robertson, (1987) and Evans, (1988). The C region encodes the DNA-binding domain, the E region encodes the ligand-binding domain and the F region encodes the carboxy-terminus domain. The D region is believed to function as a "hinge". The function of the A/B (or N-terminus) region is not entirely clear; it may be involved with enhancement and repression of receptor transcription activity. See for example, Hollenberg, et al., (1988) and Oro, et al., (1988).
The present specification also discloses chimeric receptors made by "swapping" functional domains between the new gamma retinoic acid receptor and the glucocorticoid, the mineralocorticoid, the progesterone, the estrogen, the estrogen-related (ERR1 and ERR2), the vitamin D.sub.3 receptor, the thyroid receptors, the v-erb-A receptor, the E75 (Drosophilia) receptor and the alpha and beta retinoic acid receptors. These chimeric receptors have hybrid functional characteristics based on the "origin" of the "parental" DNA-binding and ligand-binding domains incorporated within the chimeras. For example, if the DNA-binding domain in the chimeric receptor is the gamma retinoic acid receptor DNA-binding domain (i.e., is obtained from wild-type gamma retinoic acid receptor or is a mutant that contains the functional elements of the gamma retinoic acid DNA-binding domain), then the chimera will have DNA-binding properties characteristic of the gamma retinoic acid receptor. The same is true of the ligand-binding domain.