Biology Reference
In-Depth Information
Other accessory factors with infl uence on the microprocessor
complex and pri-miRNA processing are adenosine deaminases acting
on RNA (ADAR), ARS2 (a component of the nuclear RNA cap-
binding complex), estrogen receptor
), Ewing sarcoma break-
point region 1 (EWSR1), fused in sarcoma (Fus), KH-type splicing
regulatory protein (KHRSP), numerous heterogenous nuclear RNA
complex (hnRNP) proteins, nuclear factor 90 and nuclear factor 45
complex (NF90-NF45 complex), TAR DNA-binding protein-43
(TDP-43), cardinal tumor suppressor protein 53 (p53), and 396-
amino acid long SMAD nuclear interacting protein 1 (SNIP1) [
19
-
27
]. ADAR for example has a prominent role in miRNA editing of a
subset of pri-miRNA transcripts as it modifi es adenosine (A) to ino-
sine (I) by deamination. Inosine shares similar base properties with
guanine (G). A-to-I editing can change the actual miRNA precursor
sequence and therefore its base-pairing, structural, and target recog-
nition properties [
28
]. A-to-I editing has been described for miR-22,
miR-151, miR-197, miR-223, miR-376a, miR-379, miR-99a, miR-
142, miR-223, miR-1-1, and miR-143 [
19
,
29
,
30
].
Drosha cofactor DGCR8 has two double-stranded RNA
(dsRNA)-binding sites and is pivotal for Drosha-mediated cut-
ting of the pri-miRNA transcript as it specifi cally facilitates bind-
ing to the pri-miRNA cutting site of Drosha [
31
]. The pri-miRNA
contains a dsRNA hairpin stem of ~33 base pairs and a terminal
loop with imperfect complementarity and fl anking single-stranded
RNA (ssRNA) at its base. DGCR-8 recognizes the ssRNA/
dsRNA junction of the pri-miRNA. The transcription of the pri-
miRNA transcript is followed by the generation of pre-miRNAs.
The pri-miRNA is cut by the two RNase domains of Drosha at
the ssRNA/dsRNA junction, about 11 nt away from the pri-
miRNA hairpin base resulting in a pre-miRNA with a small 3
α
(ER
α
′
overhang as a result of asymmetrical cleavage of dsRNA by the
microprocessor complex [
32
,
33
]. Homeostatic control of
miRNA biogenesis is achieved through crossregulation between
Drosha and DGCR8. DGCR8 stabilizes the Drosha protein via
protein-protein interaction between its conserved carboxy-ter-
minal domain and the middle domain of Drosha [
34
]. Drosha in
turn cleaves two hairpin structures embedded in the DGCR8
mRNA [
35
]. Cleavage of the two hairpins in the DGCR8 mRNA,
one in the 5
untranslated region (UTR) and the other in the
coding region close to the start of the open reading frame
(ORF), leads to its destabilization and decreased DGCR8 expression
[
33
,
36
]. This posttranscriptional regulation of DGCR8 by the
microprocessor is an autoregulatory, negative feedback mecha-
nism controlling the microprocessor. Interestingly deep-sequenc-
ing analyses have shown that DGCR8 mRNA is the only mRNA
which is targeted by the microprocessor [
37
].
The resulting product of the fi rst microprocessor-mediated
cutting event is called pre-miRNA, is 60-100 nts in length, and has
a characteristic two-nucleotide overhang at the 3
′
′
terminal end.
