Molecular characterization of three forms of putative membrane-bound progestin receptors and their tissue-distribution in channel catfish, Ictalurus punctatus.

Membrane-bound progestin receptors (mPR) were recently cloned and characterized as a new class of steroid receptors that transduce cell-signals through alteration of MAP kinase- and cAMP-dependent pathways. To further develop our understanding of this new class of steroid receptors, we characterized the cDNAs and genes of thealpha, beta and gamma forms of the channel catfish mPRs (IpmPR). The predicted alpha and beta proteins have 49% sequence identity, whereas they only have 30% and 27% identities, respectively, with the gamma form. Furthermore, IpmPRalpha and IpmPRbeta genes have similar structures featuring intronless coding regions, while IpmPRgamma gene is composed of 8 exons and 7 introns. The 5'-flanking region of each IpmPR gene differs, but each contains putative transcriptional regulatory elements of factors known to influence reproductive physiology and endocrine disruption, for example, responsive elements for cAMP and steroids and the recognition sites for steroidogenic factor-1 and for the aryl hydrocarbon receptor. The IpmPRalpha gene was detected in all the tissues tested with relatively greater expression in brain, pituitary, muscle and testis. The expression of IpmPRbeta was much lower than that of IpmPRalpha and the transcript was predominantly observed in brain, pituitary, ovary and testis. In contrast, the IpmPRgamma transcript was mainly detected in gill, ventral aorta, intestine, and trunk kidney. In conclusion, all the structural features of the IpmPRs strongly suggest that the closely related alpha and beta forms are distantly related to the gamma form. Additionally, regulatory features of the 5'-flanking regions and the differences in tissue-specific expression of each IpmPR gene suggest that they are involved in different endocrine functions in catfish.