September 27, 2004 - Chromosomes, with their distinct morphologies
and well-known function as the storehouses of genomic DNA, are one of the
most familiar structures in the biology of the cell. In eukaryotes, these
organelles are manufactured from complexes of DNA, histones and other proteins,
called chromatin. This complex organization spools and folds lengthy strands
of genetic material into compact aggregates capable of fitting within the
tiny space within the nucleus while fulfilling their function as depots
and providers of information used by the cell's transcription machinery.
The ability to condense itself so that genes needed for protein production
remain accessible, while others remain knotted into the deeper recesses
is central to chromatin organization, and is evidenced in the existence
of two structurally different forms - a less condensed form called euchromatin
and its more densely-packed counterpart, heterochromatin. In most contexts,
heterochromatic regions do not permit the expression of any genes they
might contain, owing to their extreme compactness and epigenetic modifications
Heterochromatin is, nonetheless, important to the organization of non gene-encoding
chromosomal regions necessary to the cell's ability to survive and replicate.
| Scheme of chromodomain protein
structure and expression
The chromosomes of the fission yeast, Schizosaccharomyces pombe , contain
a number of heterochromatic regions, including centromeres, telomeres and the
mating-type region. The configuration and function of chromatin in these domains
is studied as a model of how chromatin organization achieves its repressive
effects in other species, including our own. Generally, the formation of higher-order
chromatin structure can be divided, at least, into two processes; the establishment
and maintenance. Researchers at the RIKEN Center for Developmental Biology (CDB;
Kobe, Japan) Laboratory
for Chromatin Dynamics (Jun-ichi Nakayama, Team Leader) now report the identification of a role for the protein, Chp1, in the establishment
of heterochromatin. This protein had previously been implicated as important
to chromatin organization by studies that showed that yeast lacking the chp1 gene
suffered defects in chromosome segregation and centromeric transcriptional silencing,
and that the Chp1 localizes to centromeric heterochromatic regions. It is also
related to other known chromatin assembly molecules by virtue of its possession
of a conserved motif, known as the chromodomain, shared by many of the protein
players involved in epigenetic control of gene expression. Different chromodomain-containing
proteins have been thought to play discrete roles in the chromosome's various
Nakayama and colleagues started to analyze the localization of three
chromodomain proteins, Chp1, Chp2, and Swi6. Swi6 is a homolog of heterochromatin
protein 1 (HP1) in mammals, and has been shown to play a crucial role
in the formation of heterochromatin. The researchers found that Chp1 does
indeed associate with all three heterochromatic regions in S. pombe ,
the first demonstration of its presence outside of the centromere . A
detailed analysis of the differences in the localization patterns of Swi6
and Chp2 in mutant strains of fission yeast lacking Chp1 suggested that
this protein plays a vital role in the localization of Swi6 and Chp2 specifically
to the centromeric heterochromatin.
This sparked the team's interest in whether its function might be linked
to RNA interference (RNAi), a nearly universal process responsible for
post-transcriptional gene silencing and known to be linked to both the
establishment of heterochromatin and to centromere-specific gene silencing
activity in fission yeast. Their further experiments showed that loss
of chp1 function had similar effects on the accumulation of
RNA transcripts to that of the loss of RNAi machinery components, indicating
a role in either the production or processing of centromeric RNA, either
of which might involve heterochromatin establishment. These similarities
suggest that RNAi machinery and Chp1 work together; however, it remained
unclear why mutations in chp1 or RNAi cause centromere-specific
defects. Given Chp1's universal association at centromeres, telomeres
and the mating-type region, they reasoned that the protein must have common
function in all heterochromatic domains.
Histone proteins in chromatin, heterochromatin in particular, are subject
to a form of epigenetic modification called methylation, which affects
the expression of genes within the methylated region, generally by inactivating
them. The introduction of methyl modification on histones is thought to
be an initial and critical step in the establishment of heterochromatin.
Nakayama and colleagues designed experiments to analyze the establishment
steps using the histone methyltransferase, Clr4. When the function of
the gene, clr4 , is disturbed, methylation is drastically reduced;
reintroduction of clr4 restores this defect. However, in a chp1 mutant,
the restoration of clr4 failed to re-establish methylation not
only at centromeres, but also at the mating-type region and telomeres.
These experiments elegantly demonstrated the common function of Chp1 in
the establishment of all heterochromatic domains. Interestingly, tests
of the chromodomain proteins Swi6 and Chp2 revealed that both were essential,
possibly overlapping, factors in the maintenance of H3-K9 methylation.
They concluded that these three chromodomain proteins play distinct and
cooperative roles in the establishment and maintenance of heterochromatin
This study reveals a surprising, almost confounding, intricacy and specialization
of epigenetic function that belies the seeming simplicity of S. pombe .
The region-specificity and diversity of the activities of different proteins
on the establishment and maintenance of methylation in the yeast chromosome
underscores the importance of gene silencing to - for what non-critical
function would evolve such elaborate machinery? - and highlights the dazzling
complexity that characterizes even seemingly simple biological systems.
Questions, of course, remain for further study. It remains to be seen whether
higher eukaryotes use similar mechanisms to establish and maintain heterochromatin
structure, and if so, whether counterparts for Chp1 exist in these species.
Whatever the answers may be, studies of fission yeast will continue to
be needed to help develop a better understanding of the molecular mechanisms
underlying chromatin-based epigenetic phenomena.