Residue threonine 350 confers steroid hormone responsiveness to the mouse nuclear orphan receptor CAR.

Steroid hormones modulate activity of the nuclear receptor constitutive active receptor (CAR, or constitutive androstane receptor) in mouse liver. Progesterone and testosterone repress the constitutive activity of mouse CAR (mCAR) in cell-mediated transfection assays, whereas estrogens activate the repressed receptor. This repression and activation is not observed with human CAR. To define the structural basis that confers the hormone responsiveness to mCAR, we constructed various chimeric and mutated receptors and examined their response to steroid hormones. The hormone responsiveness resided near or within AF-2 domain of mCAR. Moreover, a single mutation of threonine at position 350 to the corresponding methionine in the human counterpart abolished the repression of mCAR by steroid hormones. Coactivation by steroid receptor coactivator 1 (SRC-1) of mCAR did not depend on the threonine 350. However, overexpression of SRC-1 counteracted progesterone to repress mCAR activity. Thus, threonine 350 seems to regulate hormone responsiveness of mCAR by interfering indirectly an interaction of the receptor with a coactivator.