Fig. 5.

A working model depicting how BBX21 is involved in the COP1-HY5 signaling hub. In darkness, activated COP1 directly targets downstream substrates including BBX21, HY5, and HYH for ubiquitination, which subsequently triggers their proteolysis via the 26S proteasome system. On light illumination, the activity of COP1 is inhibited, resulting in the accumulation of BBX21, HY5, and HYH. Accumulated BBX21, HY5, and HYH directly bind to the T/G-box, and CAM7 associates with the E-box in the HY5 promoter. Together, these factors induce HY5 expression, leading to a high abundance of HY5, which regulates a large number of genes involved in promoting photomorphogenesis. ub, ubiquitination; u, ubiquitin.