We have made considerable progress in
the elucidation of the structure and function of nucleosomes and
chromatin containing the histone variants H2A.Z, H2A.Bbd,
macro-H2A, and the centromeric H3 histone variant CenpA, using
x-ray crystallography, analytical ultracentrifugation,
fluorescence resonance energy transfer, transcription assays,
and in vivo targeting and mass spectrometry.
The replacement of canonical histones with histone variants has
emerged as an important pathway to locally alter the biochemical
make-up of chromatin, with the potential to exert considerable
influence on the structure and function of chromatin. Histone
variants are distinct non-allelic forms of conventional,
major-type histones that form the bulk of nucleosomes during
replication and whose synthesis is tightly coupled to S-phase.
They are found in most eukaryotic organisms, and are expressed
in all tissue types.
One emerging theme arising from our studies is that the overall
structure of the nucleosome is by and large maintained upon
incorporation of histone variants, while subtle differences in
the surface and stability of the nucleosome, or in the way in
which the ends of the DNA are being organized by the histone
octamer differ between major-type and variant histones. For
example, in H2A.Z, surface changes result in changes in
higher-order structure formation that are ultimately important
for development. In other instances (macroH2A), extra-nucleosomal
non-histone domains change local chromatin structure by
recruiting histone modifying enzymes and other non-histone
chromatin associated proteins. We are also investigating the
mechanisms by which histone variants are being incorporated into
chromatin in a replication-independent manner. These studies
have been supported by the
Human Frontiers Science Organization,
by the March of Dimes Birth Defects Organization, and by the
NIH.
