An alternatively spliced rat mineralocorticoid receptor mRNA causing truncation of the steroid binding domain

We attempted to clone the putative 11-dehydrocorticosterone receptor by RT-PCR with two degenerate primers from highly homologous regions of the DNA and steroid binding domains of the receptor subfamily. In doing so, we have identified an alternatively spliced variant mRNA of the rat mineralocorticoid (MR) with a ten bp deletion in the C-terminal steroid binding domain. This deletion results in a truncated MR receptor of 807 amino acids in comparison to the wild type of 981 amino acids. The deletion variant was expressed in colon, kidney, heart, liver, aorta and brain tissues. The relative abundance of the deletion variant compared to the wild type MR was estimated to be 6% in rat kidney and 4% in hippocampus. This deletion was also detected in human kidney by RT-PCR. Site-directed mutagenesis was used to create the eukaryotic expression plasmid pCR3-rMRdel10 from the wild type for a transactivation assay using the luciferase reporter system in CV-1 cells. The deletion variant had the same baseline transactivation activity as the wild type MR, but did not respond to aldosterone or corticosterone stimulation. Co-transfection of MR with the deletion variant had no significant effect on transactivation activity of the MR, indicating that the deletion variant is unlikely to serve as a negative regulator of MR function.