A Novel Heme-Regulatory Motif Mediates Heme-Dependent Degradation of the Circadian Factor Period 2

摘要

Although efforts have been made to identify circadian-controlled genes regulating cell cycle progression and cell death, little is known about the metabolic signals modulating circadian regulation of gene expression. We identify heme, an iron-containing prosthetic group, as a regulatory ligand controlling human Period-2 (hPer2) stability. Furthermore, we define a novel heme-regulatory motif within the C terminus of hPer2 (SC⁸⁴¹PA) as necessary for heme binding and protein destabilization. Spectroscopy reveals that whereas the PAS domain binds to both the ferric and ferrous forms of heme, SC⁸⁴¹PA binds exclusively to ferric heme, thus acting as a redox sensor. Consequently, binding prevents hPer2 from interacting with its stabilizing counterpart cryptochrome. In vivo, hPer2 downregulation is suppressed by inhibitors of heme synthesis or proteasome activity, while SA⁸⁴¹PA is sufficient to stabilize hPer2 in transfected cells. Moreover, heme binding to the SC⁸⁴¹PA motif directly impacts circadian gene expression, resulting in altered period length. Overall, the data support a model where heme-mediated oxidation triggers hPer2 degradation, thus controlling heterodimerization and ultimately gene transcription.