Controls normal photoperiodic flowering and regulates circadian rhythms. Required for suppression of photomorphogenesis in dark-grown seedlings and for normal elongation growth of adult plants. Integral component of the COP1/SPA E3 ubiquitin-protein ligase complex. Involved in HY5, HFR1, LAF1 and CO degradation. Interacts with CO, COP1, HFR1, HY5 and PHYA. Light induces dissociation of the SPA1/COP1 complex. Binds to CRY1 in response to blue light, this interaction prevents SPA1/COP1 complex formation but stimulate CRY2/COP1 complex, and thus avoid COP1-dependent degradation of the transcription factor HY5 by the proteasome and promotes hypocotyl elongation (PubMed:21514160, PubMed:21511872, PubMed:21511871, PubMed:22739826). Present in nuclear bodies (NBs). By PHYA under continuous far-red light. Circadian-regulation under constant light. Up-regulated by red, far-red and blue light, and during the night phase under short-day conditions. The coiled-coil domain (557-589) and the WD-repeat domain are sufficient for SPA1 function. The coiled-coil domain is necessary and sufficient for interactions with HFR1 and COP1. The protein kinase domain may play a role in promoting destabilization of SPA1 under far-red light. The DWD box is required for interaction with DDB1A (By similarity). Reduced hypocotyl elongation in blue light. Was originally thought to correspond to two different genes At2g46340 and At2g46350. Was originally thought to correspond to two different genes At2g46340 and At2g46350.