Purification of Chlamydomonas 28-kDa ubiquitinated protein and its identification as ubiquitinated histone H2B

One of the most predominantly ubiquitinated protein species in Chlamydomonas, of which the apparent molecular mass in SDS-PAGE was 28 kDa, was found to exist abundantly in nuclei. The 28-kDa ubiquitinated protein was purified to homogeneity from the isolated nuclei of Chlamydomonas, and its partial amino acid sequence was determined. The N-terminal peptide sequence was identical with that of ubiquitin. Sequences homologous to those Chlamydomonas ubiquitin [corrected] and wheat histone H2B, and paired sequences of both of them were found in arginylendopeptidase-digested or protease V8-digested polypeptide fragments of the 28-kDa ubiquitinated protein. Based on these results, it was concluded that Chlamydomonas 28-kDa ubiquitinated protein is monoubiquitinated histone H2B.