Structurally unique basic protein coextracted with histones from calf thymus chromatin.

A histone-like protein that is rich in alanine and lysine (protein AK) has been obtained in homogeneous form by high-resolution gel filtration of H2SO4 extracts of calf thymus chromatin. Protein AK: (i) migrates as a single band in polyacrylamide gel electrophoresis in both acetic acid/urea and sodium dodecyl sulfate; (ii) is a basic protein; (iii) lacks tryptophan; (iv) is not extracted from chromatin with 0.35 M NaCl; and (v) is not soluble in 0.75 M HClO4. Protein AK is distinguished from the high-mobility group (HMG) proteins on the basis of these latter solubility characteristics and from the histones and protein A24 on the basis of amino acid composition and distribution of tryptic peptides in two-dimensional chromato-electrophoresis. In addition, protein AK is distinguished from the HMG proteins, the histones, and protein A24 on the basis of its mobility in two polyacrylamide gel electrophoresis systems. The amino acid composition of protein AK resembles that of HU, a basic DNA-binding protein found in Escherichia coli.