Light- and calcium-modulated phosphorylation of proteins from wheat seedlings

Abstract

In vivo white light irradiation (for 1 hr or more) of four-day-old etiolated wheat seedlings followed by in vitro phosphorylation decreased the phosphorylation of 52 and 48 kDa polypeptides in the proteins of the soluble fraction; short pulses of white light or red/far-red were not effective. Studies using norflurazon, a bleaching herbicide, suggest that dephosphorylation of this polypeptide may be linked with light-dependent development of plastids. Studies employing a Ca²⁺ chelator, EGTA, and several calmodulin (CaM) inhibitors indicate that phosphorylation of 52, 48, 34 and 31 kDa polypeptides, both in the dialysed and undialysed soluble fractions, is Ca²⁺---CaM dependent. The depletion of Ca²⁺ also retarded the mobility of a 52 kDa polypeptide by 4-6 kDa, particularly in the undialysed fraction, which was restored to control level by increasing the Ca²⁺ level, a property unique to Ca²⁺-binding proteins. Strikingly, the phosphorylation status of a doublet (17 and 15 kDa phosphopolypeptides), visible primarily in the dialysed fraction, was not affected by light and/or Ca²⁺---CaM antagonists. The results suggest the existence of Ca²⁺/CaM-dependent protein kinase(s) in young wheat seedlings, whose activity, directly or indirectly, is down-regulated by white light.