Phosphorylation of linker histones by cAMP-dependent protein kinase in mitotic micronuclei of Tetrahymena.

Linker histones (LHs) in transcriptionally inactive, mitotically dividing micronuclei of Tetrahymena thermophila, alpha, beta, gamma and delta, are highly phosphorylated in vivo. Analysis of the derived sequences of these LHs suggests that none of these polypeptides contain sites of phosphorylation by p34cdc2, the kinase thought to play an essential role governing the entry of all cells into mitosis. Surprisingly alpha, beta, gamma and delta each contain sites for phosphorylation by cyclic AMP-dependent kinase (PKA). p34cdc2 kinase phosphorylases H1 in vitro but fails to phosphorylate alpha, beta, gamma and delta. Conversely, PKA phosphorylates each of the micronuclear LHs but is unable to phosphorylate macronuclear H1. Micronuclear LHs labeled in vivo with [32P]phosphate were purified by reverse phase HPLC. Phosphoamino acid analysis showed that all four micronuclear LHs are phosphorylated exclusively on serine residues in vitro. Cyanogen bromide mapping of alpha, beta, gamma and delta labeled in vivo or in vitro by PKA indicates that each LH is phosphorylated only on peptides that contain either optimum (RR/KXS) or less optimum (RXXS) PKA sequences. This study suggests that PKA or a PKA-like activity(ies), but not p34cdc2 kinase, is(are) responsible for the in vivo phosphorylation of LHs in the mitotic micronucleus of Tetrahymena. We suggest that, at least in Tetrahymena, PKA-driven phosphorylation or dephosphorylation plays a significant role in the control of mitotic processes such as chromosome condensation.