Biology Reference
In-Depth Information
is now substantial evidence arguing that HP1 can promote transcription
(
Kwon & Workman, 2011
). The mammalian HP1
b
has been found to play
a role in the control of the expression of rDNA genes by RNA polymerase I
(RNAPI) in a Suv39h-dependent manner (
Hor´kov´ et al., 2010
), whereas
HP1
g
regulates transcriptional elongation through its association with RNA
polymerase II (RNAPII) (
Vakoc, Mandat, Olenchock, & Blobel, 2005
) and
RNA processing (
Smallwood et al., 2012
). Moreover, HP1
g
is part of a
complex with the Polycomb group protein L3mbtl2, forming a PRC1-like
complex involved in gene repression in mouse ESC and early embryogenesis
(
Qin et al., 2012; Trojer et al., 2011
).
Interestingly,
Drosophila HP1a (Su(var)2
-
5),
which is required for fly
development (
Eissenberg et al., 1992; Kellum & Alberts, 1995; Kellum
et al., 1995
), may share some function in RNA processing as it was found
to positively regulate euchromatic gene expression by interacting with the
heterogeneous ribonucleoprotein DDP1, HRB87F, and PEP (
Piacentini
et al., 2009
). In
Drosophila
, HP1 isoforms are also involved in gene regula-
tion. Indeed, HP1a, positively regulates (indirectly or not) heterochromatic
as well as euchromatic genes, like heat-shock (
Hsp70
) or cell-cycle related
genes (
Mcms
,
ORC4
,
CAF-1
,
Cdc45
, and
Aurora B
). More generally, HP1a
is a positive regulator of transcription by facilitating H3K36 demethylation
via chromoshadow domain (CSD)-mediated recruitment of dKDM4A at
active and/or heterochromatic regions (
Lin, Paulson, Abmayr, &
Workman, 2012; Lin et al., 2008
). Either knockdown or overexpression
of
HP1b
is lethal for fly development, where it seems to play an important
role in transcription. HP1c associates with the transcription factors WOC
(
without children
) and ROW (
related of WOC
) via their PxVxL motif
to regulate a common set of genes involved in nervous system development
(
Abel, Eskeland, Raffa, Kremmer, & Imhof, 2009; Font-Burgada, Rossell,
Auer, & Azor´n, 2008
). Moreover, HP1c appears also to be required for
proper
recruitment of FACT (facilitates chromatin transcription)
to
chromatin (
Kwon et al., 2010
).
In addition to fly and mammals, both isoforms of HP1 in
C. elegans
(HPL-1, HPL-2) show preferential euchromatic localization,
HPL-2
mutants showed that it is required for the regulation of germline genes, as
well as sets of genes involved in lipid metabolism or innate immunity
(
Couteau, Guerry, Muller, & Palladino, 2002; Meister et al., 2011;
Studencka et al., 2012
). The only HP1 homologue identified in
Arabidopsis
thaliana
(LHP1/TLF2) also localizes to euchromatic domains, where it asso-
ciates with PcG proteins to represses genes involved in plant development
