Biochemical characterization of 60S acidic ribosomal P proteins associated with CK-II from bamboo shoots and potent inhibitors of their phosphorylation in vitro

Abstract

Three effective phosphate acceptors (35, 15 and 13 kDa polypeptides) for casein kinase II (CK-II) in the Superdex CK-II fraction prepared from a 0.5 M NaCl extract of bamboo shoots were selectively purified by glycyrrhizin (GL)- affinity column chromatography (HPLC). These three proteins (p35, p15 and p13) were identified as 60S acidic ribosomal P proteins by determination of their partial N-terminal sequences. CK-II was associated with p35 since the GL-affinity fraction was coprecipitated with an anti-serum against Drosophila CK-IIβ. Moreover, a derivative (oGA) of glycyrrhetinic acid (GA) and several polyphenol-containing anti-oxidative compounds [quercetin, epigallocatechin gallate (EGCG) and two isoflavones, i.e., 3',4',7-trihydroxyisoflavone (3',4',7-THI) and 8-chloro-3',4',5,7-tetrahydroxyisoflavone (8C-3',4',5,7- THI)] inhibited the CK-II-mediated phosphorylation of 60S acidic ribosomal P proteins in vitro. Quercetin was found to be the most effective compound on CK-II activity since its ID50 was approx. 50 nM. These results suggest that (i) GL-affinity column chromatography is useful for the selective purification of 60S acidic ribosomal P proteins as a heterocomplex associated with CK-II from various cell sources; (ii) natural anti-oxidative compounds with polyphenols, but not GL and GA, act as potent CK-II suppressors; and (iii) CK-II mediates the regulation of the physiological functions of 60S acidic ribosomal P proteins in growing plant cells.