Biology Reference
In-Depth Information
protein complexes containing both wild-type and mutant ADAR proteins
demonstrated that homodimerization of ADAR proteins occurs independently
of RNA binding (Valente and Nishikura, 2007). However, the dimeric ADAR
proteins containing one mutant and one wild-type monomer behaved in a
dominant negative manner in both an RNA-binding assay and an
editing
assay indicating that RNA binding of both monomers is required for deamina-
tion to occur.
The conflicting reports likely arise from the different experimental
models, whether the experiments were performed
in vitro
in cell culture.
Also, posttranslational modification could be involved as some proteins were
expressed in yeast and others in Sf9 insect cells. Furthermore, proteins from
different species such as
in vitro
or
in vivo
and human ADARs were used.
In yeast, ADAT2 and ADAT3 have been shown to function as a
heterodimer where ADAT2 provides the catalytic function and ADAT3 pro-
vides substrate specificity (Gerber and Keller, 1999). This raised the possibility
that ADARs may function as heterodimers. Heterodimerization between ADAR
proteins may reveal a role for ADAR3, where ADAR1 or ADAR2 provides the
catalytic activity and ADAR3 affects the substrate specificity. FRET analysis
indicates that heterodimers between ADAR1 and ADAR2 monomers form
in vivo
Drosophila
, 2006) and heterodimers between ADAR1 and
ADAR2 were co-immunoprecipitated from astrocytoma cell lines, where it was
demonstrated that increased levels of ADAR1 can inhibit editing of substrates by
ADAR2 (Cenci
(Chilibeck
et al.
, 2008). It remains to be seen whether overexpression of
ADAR3 can have the same effect. Dimers formed between the two isoforms of
ADAR1 (p110 and p150) are readily detectable indicating that the Z-DNA
binding domains absent in the p110 isoform are not required for dimerization
(Cho
et al.
et al.
, 2003).
VI. SECONDARY STRUCTURES OF RNA-EDITING SUBSTRATES
Initial investigations into
site editing revealed that a complementary
editing site complementary sequence (ECS) located in the downstream intron
was required for editing to occur (Higuchi
GluR-BQ/R
, 1993; Fig. 3.1). These sequences
form an imperfect stem-loop structure which is bound by the dsRBDs of
ADAR2. Point mutations which destabilized the stem-loop decreased the editing
frequency, but this could be restored by making the complementary mutation in
the opposite base, indicating it is the structure not the sequence that is important.
Mammalian ADAR1 and ADAR2 edit some of the same sites yet other
sites are clearly specific for one or other protein, raising questions about substrate
recognition. While there is no consensus sequence surrounding edited adenosine
et al.
