Fig. 2. (A) Domain structure and covalent modifications of human SPAK and OSR1 protein kinases. Residues on SPAK and OSR1 that are phosphorylated by WNK1 and WNK4 are highlighted in red. Further phosphorylation sites are in black and are reported on the PhosphoSitePlus website (http://www.phosphosite.org). (B) Crystal structure of the CCT domain of OSR1 in complex with a WNK4-derived RFxV-containing peptide. (Left panel) Sequence conservation of the protein surface of the CCT domain of OSR1 in orthologues from C. elegans to human. Grey represents non-conserved residues and red represents identical residues. The location of the primary pocket, in which the RFxV motif (peptide stick representation with yellow carbon atoms) is bound, and the surface-exposed secondary groove (of unknown function) are labelled. (Right panel) Molecular interactions of the CCT domain of OSR1 (-helices are in red and β-strands in blue) and the WNK4-derived RFQV-containing peptide. The RFQV motif (residues 1016-1019) and residue Thr1020 are labelled. The arrow indicates that phosphorylation of Thr1020 would cause a steric clash with the backbone of the CCT domain, which could inhibit binding of the RFQV motif. Note that Thr1020 is equivalent to the residue termed Thr1008 in the original structural study (Villa et al., 2007), which was wrongly numbered.