May act as a substrate-specific adapter of an E3 ubiquitin-protein ligase complex (CUL3-RBX1-BTB) which mediates the ubiquitination and subsequent proteasomal degradation of target proteins (By similarity). Signal transducer of the phototropic response and photo-induced movements. Necessary for root phototropism. Involved in hypocotyl phototropism under high rate but not under low rate light. Regulates stomata opening. Seems to be not involved in chloroplast accumulation and translocation. Protein modification; protein ubiquitination. Interacts with RPT3 and PHOT1. A number of isoforms are produced. According to EST sequences. Expressed in hypocotyls, guard cells and mesophyll cells. Light fluence rate-dependent induction, independent of light quality. Up-regulated by blue light treatment. The BTB/POZ domain mediates the interaction with some component of ubiquitin ligase complexes. Belongs to the NPH3 family. Was originally thought to correspond to two different genes At2g30510 and At2g30520. Was originally thought to correspond to two different genes At2g30510 and At2g30520.