The N-terminal region of human progesterone B-receptors - Biophysical and biochemical comparison to A-receptors

To understand the basis for functional differences between the two human pr ogesterone receptors (PR), we have carried out a detailed biochemical and b iophysical analysis of the N-terminal region of each isoform, Extending our previous work on the A-isoform (Bain, D, L, Franden, M, A, McManaman, J, L ,, Takimoto, G, S,, and Horwitz, R, B, (2000) J, Biol, Chem, 275, 7313-7320 ), here we present studies on the N-terminal region of the B isoform (NT-B) and compare its properties to its A-receptor counterpart (NT-A), As seen p reviously with NT-A, NT-B is quantitatively monomeric in solution, yet unde rgoes N-terminal-mediated assembly upon DNA binding. Limited proteolysis, m icrosequencing, and sedimentation analyses indicate that the B-isoform exis ts in a non-globular, extended conformation very similar to that of NT-A, A dditionally, the 164 amino acids unique to the B-isoform (BUS) appear to be in a more extended conformation relative to sequences common to both recep tors and do not exist as an independent structural domain. However, sedimen tation studies of NT-A and NT-B show differences in the ensemble distributi on of their conformational states. We hypothesize that isoform-specific fun ctional differences are not due to structural differences, per se. Rather, the transcriptional element BUS, or possibly other transcription factors, c auses a redistribution of the conformational ensemble by stabilizing a more functionally active set of conformations in NT-B.