Cell cycle dependent alterations of chromatin structure in situ as revealed by the accessibility of the nuclear protein AF-2 to monoclonal antibodies

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Abstract
We have recently described a novel nuclear antigen, AF‐2, which is related to cell cycle dependent alterations of chromatin structure. We show by two parameter flow cytometry on a cell by cell basis that the antigen is accessible to specific monoclonal antibodies only in mitotic and postmitotic early G1‐phase cells. The evaluation of nuclease susceptibility and AF‐2 antigen accessibility reveals different subcompartments of the G1 ‐phase of the cell cycle with distinct chromatin conformations. Digestion with DNase I seems to alter the chromatin structure according to concentration and this is reflected by an increase of the antigen accessibility. Chromatin in the more condensed early G1‐phase is specifically digested by lower concentrations of the enzyme than chromatin in later stages of interphase. Chromatin from cells in the late‐G1, S‐, and G2‐phases shows a higher relative resistance to DNase I and a reduced accessibility of the AF‐2 antigen to monoclonal antibodies. Nuclease S1 has a similar effect on chromatin topology, as revealed by the reaction with anti‐AF‐2 antibodies, without digestion of detectable amounts of DNA. The antigen becomes available to the antibodies in almost all cells by digestion with high concentrations of DNase I or Nuclease S1.