PMID: 3745192Sep 15, 1986Paper

The action of chymotrypsin on nucleosome cores. Histone products and conformational effects of limited digestion.

The Journal of Biological Chemistry
N L RosenbergR L Rill

Abstract

alpha-Chymotrypsin was used to probe accessible hydrophobic amino acid residues in nucleosome cores. Small amounts of chymotrypsin rapidly and selectively cleaved at leucine 20 of histone H3. Cleavage at this site caused partial unfolding of the nucleosome core at low ionic strengths indicated by a small decrease in sedimentation coefficient and increase in circular dichroism in the 265-285-nm range. Unfolding did not occur at moderate ionic strengths, probably because of more effective electrolyte screening of residual negative charge on the nucleosome core. More extensive treatment with chymotrypsin partially degraded other core histones in nucleosome cores at similar rates. The primary sites of cleavage were assigned to Leu115 of H2a, Val18 or Gln22 of H2b, and Leu10 plus Leu22 of H4. We conclude that these primary sites of chymotrypsin cleavage of the four core histones lie on or near the nucleosome core surface, while the large number of other hydrophobic histone residues located in more central sequences must be inaccessible. Extensive chymotrypsin treatment yielded a set of "limit" products approximately 80-100 residues long that were similar to the limit products of trypsin digestion. Sedimentation coefficients and circ...Continue Reading

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