Non‐histone protein 1 (NHP1) is a member of the Ku protein family which is upregulated in differentiating mouse myoblasts and human promyelocytes

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Abstract
We have previously purified and characterized a ubiquitous non-histone protein (NHP1) which has a high affinity (K d 10−11 M) for different avian vitellogenin gene sequences containing CpGs (Hughes et al. (1989) Biochemistry 28, 9137–9142; Hughes and Jost (1989) Nucleic Acids Res. 17, 8511–8520). Here we show by microsequencing that the peptides derived from the purified p75 and p85 subunits of NHP1 from HeLa cells have between 64 and 100% identity with the human Ku autoantigen. During the differentiation of human HL-60 promyelocytes there is an increase in the amount of p85 subunit protein whereas the level of the p75 subunit is unchanged. In differentiating mouse G8 myoblasts there is, however, an upregulation of both the p75 and p85 subunits and of the p85 mRNA. An inhibition of mouse myoblast differentiation by either cAMP, 3-aminobenzamide or sodium butyrate abolishes the upregulation of the p85 subunit. In G8 myoblasts chemical, or physical stress by UV light or X-rays does not upregulate the level of the p85 subunit. The possible involvement of NHP1 in the active demethylation of bifilarly methylated DNA will be discussed.