D22-Unsaturated sterols, containing a double bond at the C-22 position in the side chain, occur specifically in fungi and plants. Here, we describe the identification and characterization of cytochrome P450s belonging to the CYP710A family as the plant C-22 desaturase. Recombinant proteins of CYP710A1 and CYP710A2 from Arabidopsis thaliana and CYP710A11 from tomato (Lycopersicon esculentum) were expressed using a baculovirus/insect system. The Arabidopsis CYP710A1 and tomato CYP710A11 proteins exhibited C-22 desaturase activity with b-sitosterol to produce stigmasterol (CYP710A1, K m ¼ 1.0 mM and kinetic constant [k cat ] ¼ 0.53 min ÿ1 ; CYP710A11, K m ¼ 3.7 mM and k cat ¼ 10 min ÿ1 ). In Arabidopsis transgenic lines with CYP710A1 and CYP710A11 overexpression, stigmasterol levels increased by 6-to 32-fold. Arabidopsis CYP710A2 was able to produce brassicasterol and stigmasterol from 24-epi-campesterol and b-sitosterol, respectively. Sterol profiling analyses for CYP710A2 overexpression and a T-DNA insertion event into CYP710A2 clearly demonstrated in planta that CYP710A2 was responsible for both brassicasterol and stigmasterol production. Semiquantitative PCR analyses and promoter:b-glucuronidase transgenic approaches indicated strict tissue/organ-specific regulation for each CYP710A gene, implicating differential tissue distributions of the D 22 -unsaturated sterols in Arabidopsis. Our results support the possibility that the CYP710 family may encode P450s of sterol C-22 desaturases in different organisms.

The reversible formation of a glyceraldehyde-3-phosphate dehydrogenase (GAPDH)-CP12-phosphoribulokinase (PRK) supramolecular complex, identified in oxygenic photosynthetic organisms, provides light-dependent Calvin cycle regulation in a coordinated manner. An intrinsically disordered protein (IDP) CP12 acts as a linker to sequentially bind GAPDH and PRK to downregulate both enzymes. Here, we report the crystal structures of the ternary GAPDH-CP12-NAD and binary GAPDH-NAD complexes from Synechococcus elongates. The GAPDH-CP12 complex structure reveals that the oxidized CP12 becomes partially structured upon GAPDH binding. The C-terminus of CP12 is inserted into the active-site region of GAPDH, resulting in competitive inhibition of GAPDH. This study also provides insight into how the GAPDH-CP12 complex is dissociated by a high NADP(H)/NAD(H) ratio. An unexpected increase in negative charge potential that emerged upon CP12 binding highlights the biological function of CP12 in the sequential assembly of the supramolecular complex.

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