Identification and Mutation of Phosphorylation Sites in a Linker Histone

Abstract

Linker histone phosphorylation has been suggested to play roles in both chromosome condensation and transcriptional regulation. In the ciliated protozoan Tetrahymena, in contrast to many eukaryotes, histone H1 of macronuclei is highly phosphorylated during interphase. Macronuclei divide amitotically without overt chromosome condensation in this organism, suggesting that requirements for phosphorylation of macronuclear H1 may be limited to transcriptional regulation. Here we report the major sites of phosphorylation of macronuclear H1 in Tetrahymena thermophila. Five phosphorylation sites, present in a single cluster, were identified by sequencing 32 P-labeled peptides isolated from tryptic peptide maps. Phosphothreonine was detected within two TPVK motifs and one TPTK motif that resemble established p34 cdc2 kinase consensus sequences. Phosphoserine was detected at two non-proline-directed sites that do not resemble known kinase consensus sequences. Phosphorylation at the two noncanonical sites appears to be hierarchical because it was observed only when a nearby p34 cdc2 site was also phosphorylated. Cells expressing macronuclear H1 containing alanine substitutions at all five of these phosphorylation sites were viable even though macronuclear H1 phosphorylation was abolished. These data suggest that the five sites identified comprise the entire collection of sites utilized by Tetrahymena and demonstrate that phosphorylation of macronuclear H1, like the protein itself, is not essential for viability in Tetrahymena.In eukaryotes, the association of DNA with histones in nucleosomes and the folding of nucleosomal filaments within chromatin can restrict the accessibility of DNA sequences to factors required for gene expression and DNA replication (1, 2). Crystallographic analyses have led to detailed appreciation of the arrangement of core histones within nucleosomes and the histone-histone and histone-DNA interactions responsible for the stability of nucleosomal structure (3, 4). Many aspects of nucleosome structure are expected to be common to all eukaryotes because core histones are among the most highly conserved proteins known (1, 2). However, the structure and occurrence of proteins, collectively referred to as H1 or linker histones that bind the outer surface of nucleosomes and portions of the linker DNA extending between adjacent nucleosomes, are more variable (1, 2, 5, 6).Data from recent biochemical and molecular genetic analyses have firmly established that chromatin structure plays a fundamental role in the regulation of gene expression. Acetylation of conserved lysine residues within the amino termini of core histones, for example, is a major pathway for modulating transcriptional activity (7,8). In contrast, the function of linker histones and their various post-translational modifications in vivo remain unclear (5, 6, 9). Previous notions that H1 acts globally to repress transcription (10, 11) and that H1 phosphorylation is involved in mitotic chromosome condensation (9) are in contrast to evidence t...

Liquid chromatography mass spectrometry profiling of histones

“…It is known that its function with regard to the organization of chromatin structure is mediated by phosphorylation [59,60]. At least seven human H1 variants (five somatic and two germinal) have been identified to date and their sequences are available in the Histone Database (http://research.nhgri.nih.gov/histones/).…”

Glucocorticoid receptor-mediated chromatin remodeling in vivo

“…This phosphorylation weakens the interaction of the tails with the DNA backbone. Further evidence for the critical role of H1 phosphorylation in regulating transcription has recently been reported Mizzen et al, 1999). CyP1 gene expression requires H1 dephosphorylation in wild-type Tetrahymena (Dou et al, 1999).…”

Relationship between Autophosphorylation and Phosphorylation of Exogenous Substrates by the Human Cytomegalovirus UL97 Protein Kinase

“…32 P-labeled GST-UL97 tryptic peptides were loaded onto a 30% (wt/vol) alkaline acrylamide gel and electrophoresed overnight at 5 mA, as described previously (5,17,25). The radioactive bands were excised, and peptides were extracted into 0.1% trifluoroacetic acid (TFA) at 4°C for 3 h with vigorous shaking.…”