InterPro domain: IPR044630

In Arabidopsis, SPA1/2/3/4 play a central role in suppression of photomorphogenesis. SPA1 and SPA2 predominate in dark-grown seedlings, whereas SPA3 and SPA4 prevalently regulate the elongation growth in adult plants [ 1 ]. SPAs contain a kinase-like domain, a coiled-coil domain and the WD-repeats. SPAs and COP1 (a ring finger E3 ubiquitin ligase) can form homo- and heterodimers via their respective coiled-coil domains, and the COP1/SPA complex forms a tetramer of two COP1 and two SPA proteins [ 2 ]. The SPA proteins can self-associate or interact with each other, forming a heterogeneous group of SPA-COP1 complexes [ 3 ]. Besides recognizing substrates, both COP1 and SPA bind DDB1 in the CUL4 complex through their C-terminal WD-repeat domains. They serve as DDB1-CUL4-associated factors (DCAFs) similar to other substrate adaptors in CUL4-based E3 ligases. SPA1 interacts with photoreceptor cry2 via its kinase-like domain, with cry1 via its WD-repeat domain and with phytochromes possibly via both [ 4 ]. SPAs have also been shown to regulate the phyB-PIF4 module at high ambient temperature [ 5 ].

1. The SPA quartet: a family of WD-repeat proteins with a central role in suppression of photomorphogenesis in arabidopsis. Plant Cell 16, 2293-306
2. The functional divergence between SPA1 and SPA2 in Arabidopsis photomorphogenesis maps primarily to the respective N-terminal kinase-like domain. BMC Plant Biol. 16, 165
3. The SPA1-like proteins SPA3 and SPA4 repress photomorphogenesis in the light. Plant J. 35, 373-85
4. The activities of the E3 ubiquitin ligase COP1/SPA, a key repressor in light signaling. Curr. Opin. Plant Biol. 37, 63-69
5. SPAs promote thermomorphogenesis by regulating the phyB-PIF4 module in Arabidopsis. Development 147