Download Antagonistic role of hnRNP A1 and KSRP in the regulation of let

Antagonistic role of hnRNP A1 and KSRP in the regulation of let-7a biogenesis
Gracjan Michlewski & Javier F Cáceres
Nature structural & molecular biology, VOLUME 17, 1011-1018, AUGUST 2010
Presenter：Zih-Huei Wu
Commentator：Cheng-Chan Lu
Date/Time：2011/06/02, 16:10~17:00
Place：602, College of Medicine
Abstract
Background
MicroRNAs (miRNAs) are endogenous ~22 nt single-stranded RNAs. They regulate mRNAs
post-transcriptionally. miRNAs are involved in various biologic processes, including development,
differentiation, apoptosis, proliferation and carcinogenesis. The let-7 family of miRNAs is present in
multiple copies in different genomes. The pluripotency-promoting proteins Lin28a and Lin28b, which are
expressed in undifferentiated cells, and bind the terminal loop of let-7 precursors and block their processing,
acting at the level of Drosha and/or Dicer. Heteronuclear ribonucleoprotein A1 (hnRNP A1) is a
nucleocytoplasmic shuttling protein with roles in many aspects of mRNA metabolism. Besides, hnRNP A1
binds the terminal loop of pri-let-7a miRNA, which harbors a perfect hnRNP A1 consensus binding site
(UAGGGA/U). The splicing factor KH-type splicing regulatory protein (KSRP) has been recently shown to
be a component of both Drosha and Dicer complexes and to positively regulate the biogenesis of a subset of
miRNAs, including miR-155 and let-7.
Objective
In this study, the authors want to confirm the existence of hnRNP A1 that bind conserved terminal loops
and that can influence, positively or negatively, the processing of specific miRNAs.
Results
First, the authors used electrophoretic gel mobility shift analysis (EMSA) to analyze the interaction
between the terminal loop of pri-let-7a and hnRNP A1. By using western blot and northern blot, they
established an inverse correlation between hnRNP A1 and let-7a levels, suggesting that hnRNP A1 could be
acting as a repressor of let-7a biogenesis. Second, they confirmed hnRNP A1 blocks the Drosha-mediated
processing of let-7a by in vitro processing assay and quantitative reverse transcription PCR (qRT-PCR).
Then, the results of footprint analysis found that hnRNP A1 binds and remodels the terminal loop of
pri-let-7a-1. In recent studies, KSRP has been shown to positively regulate the biogenesis of let-7. Third,
they found that KSRP also binds to the terminal loop of pri-let-7a-1. Finally, they confirmed that KSRP and
hnRNP A1 compete for binding to the pri-let-7a-1 terminal loop by EMSA.
Conclusion
hnRNP A1 and KSRP have antagonistic roles in the post-transcriptional regulation of let-7a expression.
Reference
Gracjan Michlewski, Sonia Guil, Colin A. Semple, and Javier F. Ca´ ceres. Posttranscriptional regulation of
miRNAs harboring conserved terminal loops. Molecular Cell 32, 383–393, November 7, 2008