Abstract
A Ca(2+)-calmodulin-activated histone 3 kinase was partially purified from nuclear extracts of dividing and quiescent rat heart endothelial cells. The histone 3 phosphorylating activity was 20-100-fold higher in quiescent than in dividing cells. Base hydrolysis followed by amino acid analysis revealed that histone 3 was phosphorylated on arginine. Further investigations were conducted to determine whether phosphorylation of histone 3 also occurred in vivo. Cells were incubated for 3 h in a phosphate-free medium supplemented with [32P]phosphoric acid. It was observed that the nuclear content of arginine-phosphorylated histone 3 was considerably higher in quiescent than in dividing rat heart endothelial cells. The histone 3 arginine kinase is a component of a complex containing a Ca(2+)-dependent calmodulin-binding protein of apparent molecular mass of 85 kDa. Using polyclonal antibodies to an 85-kDa protein, also the major Ca(2+)-dependent calmodulin-binding component of the histone 3 arginine kinase from calf thymus, an immunoreactive protein of identical apparent molecular mass was found to be present in equal amounts both in dividing and quiescent cells. We propose that the 85-kDa protein is either the histone 3 arginine kinase or one of its subunits and that phosphorylation of histone 3 is involved with cell cycle exit in eukaryotes.
Highlights
Protein phosphorylation/dephosphorylation plays an important role in a number of cellular activities in eukaryotes
We propose that the 85-kDa protein is either the histone 3 arginine kinase or one of its subunits and that phosphorylation of histone 3 is involved with cell cycle exit in eukaryotes
Our knowledge of the kinases that phosphorylate histone 3 (H3) is limited to a kinase identified as a component of the HeLa cell [13], a 38-kDa chromatin-bound H3 kinase from calf thymus chromatin [14, 15], the catalytic subunit of cAMPdependent kinase used by Shibata et al [11], and a chromatinbound Ca2ϩ-CaM-activated kinase from calf thymus [16] and from mouse leukemia cells [17]
Summary
Protein phosphorylation/dephosphorylation plays an important role in a number of cellular activities in eukaryotes (reviewed in Ref. 1). The histone 3 phosphorylating activity was 20 –100-fold higher in quiescent than in dividing cells. It was observed that the nuclear content of arginine-phosphorylated histone 3 was considerably higher in quiescent than in dividing rat heart endothelial cells.
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