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Role of Polycomb Proteins in Gene Description,
Stem Cells and Human Diseases,
by Luciano Di Croce, from the Gene
Regulation, Stem cell, and Cancer Department,
ICREA, and Center for Genomic
Regulation, Barcelona, Spain.
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The DNA of eukaryotic cells is
organized in chromatin fibers,
where the nucleosome forms
the basic repeating unit.
Each nucleosome comprises 147 base
pairs of DNA, which is wrapped
in 1.8 elegant turn around an
octamer of four highly evolutionarily
conserved histone protein, which are
histone H two A, histone H two B,
histone H three, and histone H four.
These four structure, which is
often called as 11 nanometer fibers.
The histone H1 lines to deliver DNA
between two addition nucleosomes,
causing further compaction
of the chromatin fibers
into higher order structure,
often refer as a solenoid or as 30
nanometer fibers.
The analyses of the crystal
structure of the nucleosome,
which was solved in 1998 by the
group of Luger and Richmond,
reveal that on terminal a part of the histone tail
are flexible and protrude
outside from the nucleosome core.
The histone tail
undergoes a large number
of post-translational modifications.
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Some of these modifications
are illustrated in this slide.
For example, lysine can
be acetylated, methylated,
while arginine can
also be methylated.
Serine and threonine
can be phosphorylated.
Lysine can be also
to be ubiquitinated
or sumoylated and so forth.
Too many functions have been ascribed for histone modifications.
For example, acetylation
of lysine neutralizes
the positive charge at histone tail.
And two, altering
histone DNA interaction
reduces interaction
between different histones,
in addition to nucleosomes.
Moreover, as illustrated in
the bottom part of this slide,
several post transcriptional modifications of histones
generate docking side, or modulate
the affinity of nuclear proteins
for chromatin.
The specific recognition
of histone modification
is achieved by a dozen
protein domains, which
are present in a large number of
chromatin-associated proteins.
In turn, these adaptive proteins
are usually part of large protein
complexes, implicated in chromatin
remodeling, prescription,
or for further modifications
of histone tails.
