dictyNews
Electronic Edition
Volume 39, number 34
December 6, 2013
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Abstracts
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The Dictyostelium discoideum RNA-dependent RNA Polymerase RrpC
Silences the Centromeric Retrotransposon DIRS-1 Post-transcriptionally
and is Required for the Spreading of RNA Silencing Signals
Stephan Wiegand, Doreen Meier, Carsten Seehafer, Marek Malicki,
Patrick Hofmann, Anika Schmith, Thomas Winckler, Balint Földesi,
Benjamin Boesler, Wolfgang Nellen, Johan Reimegård, Max Käller,
Jimmie Hällman, Olof Emanuelsson, Lotta Avesson,
Fredrik Söderbom and Christian Hammann
Nucleic Acids Research, in press
DIRS-1 is the founding member of a poorly characterized class of
retrotransposable elements that contain inverse long terminal repeats
and tyrosine recombinase instead of DDE-type integrase enzymes. In
Dictyostelium discoideum, DIRS-1 forms clusters that adopt the function
of centromeres, rendering tight retrotransposition control critical to
maintaining chromosome integrity. We report that in deletion strains of
the RNA-dependent RNA polymerase RrpC, full length and shorter
DIRS-1 mRNAs are strongly enriched. Shorter versions of a hitherto
unknown long non-coding RNA in DIRS-1 antisense orientation are
also enriched in rrpC- strains. Concurrent with the accumulation of long
transcripts, the vast majority of small (21mer) DIRS-1 RNAs vanish in
rrpC- strains. RNASeq reveals an asymmetric distribution of the DIRS-1
small RNAs, both along DIRS-1 and with respect to sense and antisense
orientation. We show that RrpC is required for post-transcriptional
DIRS-1 silencing, and also for the spreading of RNA silencing signals.
Finally, DIRS-1 mis-regulation in the absence of RrpC leads to
retrotransposon mobilization. In summary, our data reveals RrpC as
a key player in the silencing of centromeric retrotransposon DIRS-1.
RrpC acts at the post-transcriptional level and is involved in the
spreading of RNA silencing signals, both in the 5’ and 3’ directions.
Submitted by Christian Hammann [[log in to unmask]]
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A CRIB domain mediates functions of coronin
Karthic Swaminathan, Annette Müller-Taubenberger, Jan Faix,
Francisco Rivero, Angelika A. Noegel
PNAS, in press
The CRIB (Cdc42- and Rac-interactive binding) motif of coronin binds to
Rho GTPases with a preference for GDP-loaded Rac. Mutation of the
CRIB motif abrogates Rac binding. This results in increased 1evels of
activated Rac in coronin deficient Dictyostelium cells (corA-), which impacts
myosin II assembly. corA- cells show increased accumulation of myosin II
in the cortex of growth-phase cells. Myosin II assembly is regulated by
myosin heavy chain kinase (MHCK)-mediated phosphorylation of its tail.
Kinase activity depends on the activation state of the p21-activated kinase
PAKa. The myosin II defect of corA- mutant is alleviated by dominant
negative PAKa. It is rescued by wild type coronin whereas coronin carrying
a mutated CRIB failed to rescue the myosin defect in corA- mutant cells.
Ectopically expressed MHCKs affinity-purified from corA- cells show
reduced kinase activity. We propose that coronin through its affinity for
GDP-Rac regulates the availability of GTP-Rac for activation of
downstream effectors.
Submitted by Angelika Noegel [[log in to unmask]]
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[End dictyNews, volume 39, number 34]
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