Casein kinase I (CKI) is a protein serine/threonine kinase that is highly conserved from plants to animals. It performs various functions in both the cytoplasm and nucleus, such as DNA repair, cell cycle, cytokinesis, vesicular trafficking, morphogenesis and circadian rhythm. CKI proteins contain a highly conserved kinase domain responsible for catalytic activity at the N-terminus and a highly diverse regulatory domain responsible for determining substrate specificity at the C-terminus. CKI-like protein has been identified in plants, including in rice, but its function and structure have not been reported. Here, we report the 2.0 Å crystal structure of the kinase domain of CKI-like protein from rice. Although the structure adopts the typical bi-lobal kinase architecture, the length and orientation of the glycine-rich ATP-binding motif are dynamic within the CKI family. A loop between α5 and α6 (the α5–α6 loop), which was previously not detected in the CKI family because of flexibility, was clearly detected in our structure. In addition, we identified a lipase as a substrate of CKI-like protein from rice. Phosphorylation of the lipase dramatically reduced its catalytic activity, suggesting that CKI may play a role in the regulation of lipase activity.

中文翻译：

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水稻酪蛋白激酶I样蛋白的结构和功能研究

酪蛋白激酶I（CKI）是一种蛋白质丝氨酸/苏氨酸激酶，从植物到动物都高度保守。它在细胞质和细胞核中均具有多种功能，例如DNA修复，细胞周期，胞质分裂，囊泡运输，形态发生和昼夜节律。CKI蛋白包含负责N端催化活性的高度保守的激酶结构域和负责确定C端底物特异性的高度多样化的调节域。已经在植物中，包括在水稻中鉴定出CKI样蛋白，但是尚未报道其功能和结构。在这里，我们报道了水稻CKI样蛋白激酶结构域的2.0Å晶体结构。尽管该结构采用了典型的双叶激酶结构，在CKI家族中，富含甘氨酸的ATP结合基序的长度和方向是动态的。在我们的结构中清楚地检测到了C5系列中以前未在CKI系列中检测到的α5和α6之间的环（α5–α6环）。另外，我们鉴定了一种脂酶作为水稻CKI样蛋白的底物。脂肪酶的磷酸化显着降低了其催化活性，表明CKI可能在脂肪酶活性的调节中起作用。