Human mineralocorticoid receptor A and B protein forms produced by alternative translation sites display different transcriptional activities

Objective: Aldosterone binds the mineralocorticoid receptor (MR) and is involved in the regulation of ionic transport, mainly sodium retention. MR is encoded by one single gene transcribed into various messengers that are thought to be translated into a unique 107 kDa protein. The aim of this study was to identify and characterize translation initiation variants of the human MR protein. Methods: For this purpose we analyzed the extreme N-terminal fragment of various MR species, and confronted the predicted pattern of expression with in vitro translation results. Transactivation functions of the identified MR forms obtained by targeted mutagenesis were evaluated by transient transfection assays with different promoter reporter genes. Results: Two major forms of human MR (hMR), named MRA and MRB, generated by alternative initiation of translation from start methionine 1 and 15 respectively, were predicted. Their expected expressions were confirmed by in vitro transcription/translation studies. Interestingly, each form of MR displayed reduced transactivation capacities with wild type (WT) = A + B > MRA > MRB suggesting that the extreme N-terminal fragment of MR has an effect on the transcriptional properties of the receptor. Conclusions: Altogether these data indicate that MR is expressed, through alternative translation initiation, as distinct protein variants which possess different functional properties. These MR forms could tightly dictate the modulation of aldosterone responsiveness in various tissues or in pathophysiological situations.