We have used immunocytochemistry and molecular cloning methods to identify and characterize structural
polypeptides of the centromere. These studies permit us to resolve two distinct regions: the inner and outer centromere. (i) Components of the outer centromere:
autoantibodies from certain patients with
rheumatic disease identify a family of three immunologically related
polypeptides that we have designated CENP-
A (17 kDa), CENP-B (80 kDa), and
CENP-C (140 kDa). CENP-B has been cloned and sequenced. DNA sequence analysis indicates that this
polypeptide possesses two large regions with extraordinary concentrations of acidic residues (region I: 61 residues with 79%
glu + asp; region II: 31 residues with 87%
glu + asp). Despite this concentration of negative charge, immunocytochemical experiments suggest that CENP-B may be
a DNA binding protein. In these experiments, the levels of CENP-B are seen to vary reproducibly from chromosome to chromosome. The role of CENP-B in vivo is unknown. However, it is unlikely to bind directly to the spindle microtubules since it is found at an inactive centromere that apparently does not attach to the spindle. (ii) Components of the inner centromere: we have injected mice with the whole chromosome scaffold fraction to elicit production of
monoclonal antibodies. One such antibody identifies two structurally related
polypeptides (the INCENP
antigens, 135 and 155 kDa) that are preferentially located between the sister chromatids at the centromere. The INCENP
antigens undergo dramatic movements from the chromosomes to the central spindle during mitosis. They are ultimately sequestered in the midbody and discarded. Several lines of evidence suggest that the INCENP
polypeptides may be involved in the regulation of sister chromatid separation at the metaphase-anaphase transition.