Biology Reference
In-Depth Information
coordinate the zinc atom in the active site; and a conserved secondary structure
of alternating
-sheets. These conserved residues are organized
into three motifs: motif I (H/CxE), motif II (PC), and motif III (C), where x
indicates any amino acid. The distance between the two C motifs (motifs II and
III) varies; within the APOBEC and ADAT family it is two amino acids, whereas
within the ADAR and ADAT1 family it is more variable.
-helices and
III. THE ADAS THAT ACT ON RNA FAMILY
ADARs were originally identified as an activity in
embryo extracts
that appeared to unwind dsRNA so that it migrated differently when electro-
phoresised on a native polyacrylamide gel (Bass and Weintraub, 1987). Origi-
nally, it was thought to be a helicase activity but then it was found to modify
dsRNA so that up to 50% of the adenosine residues in the dsRNA were
deaminated to inosines (Bass, 1988, p. 3).
The ADAR family evolved from tRNA ADA (Tad1/ADAT1) by the
acquisition of several dsRNA-binding domains (dsRBDs) at the amino terminus
(Gerber and Keller, 2001; Fig. 3.2). There are four members of the ADAR family
in mammals; however, alternatively, splicing generates different isoforms with
either altered activity or cellular localization. Expression of ADAR1 and
ADAR2 is highest in the CNS, where most of the specifically edited transcripts
have been identified. ADAR3, which is catalytically inactive, is only expressed
in the brain (Melcher
Xenopus laevis
, 1996). Another member of the family known as
testis-expressed nuclear RNA-binding protein (TENR) is only expressed in the
testis (Schumacher
et al.
, 1995). It is likely to be catalytically inactive as it lacks
crucial conserved residues involved in catalysis (Connolly
et al.
et al.
, 2005).
A. ADAR1
ADAR1 was the original protein to be purified to homogeneity from different
species (Hough and Bass, 1994; Kim
, 1994; O'Connell and Keller, 1994).
The activity assay employed by different groups was the conversion of A-to-I in
long dsRNA so using this assay ADAR2 should also have been isolated. Howev-
er, as this did not occur, it suggests that ADAR2 protein is either not as abundant
as ADAR1 or is less stable. ADAR1 protein is ubiquitously expressed and is
highly conserved in many organisms but is not found in
et al.
Drosophila
; however, the
possibility exists that it has been lost from the species (Keegan
, 2004).
ADAR1 is alternatively spliced at the amino terminus to generate two
different isoforms that differ in their translation start sites and subsequently in
their subcellular localization (Kawakubo and Samuel, 2000; Patterson and
Samuel, 1995). The longer ADAR1 isoform (p150) is interferon-inducible,
et al.
