dictyNews
Electronic Edition
Volume 33, number 14
November 27, 2009
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Abstracts
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Autophagy dysfunction and Ubiquitin-positive protein aggregates in
Dictyostelium
Cells Lacking Vmp1.
Javier Calvo-Garrido and Ricardo Escalante
Instituto de Investigaciones Biomédicas Alberto Sols. C.S.I.C./U.A.M.,
Calle Arturo Duperier 4, 28029 Madrid. Spain.
Autophagy, in press
Ubiquitin-positive protein aggregates are a hallmark of many
degenerative
diseases. Their presence can be induced by dysfunction in protein
degradation pathways such as proteasome and autophagy. We now report
several lines of evidence suggesting a defect in autophagy in
Dictyostelium
cells lacking Vmp1 (Vacuole membrane protein 1), an endoplasmic
reticulum (ER)-resident protein involved in pathological processes such
as cancer and pancreatitis. vmp1- null cells are unable to survive
starvation
or undergo autophagic-cell-death under the appropriate inductive
signals.
Moreover, confocal studies using the autophagy marker Atg8 and previous
transmission electron microscopy analysis showed defects in
autophagosome formation. Although Vmp1 is localized in the ER we
found co-localization with Atg8 suggesting a contribution of both Vmp1
and ER in autophagosome biogenesis or maturation. Interestingly,
vmp1- mutant cells showed accumulation of huge ubiquitin-positive
protein
aggregates containing the autophagy marker GFP-Atg8 and the putative
Dictyostelium p62 homologue as described in many degenerative human
diseases. The analysis of other Dictyostelium autophagic mutants
(atg1-, atg5-, atg6-, atg7- and atg8-) showed a correlation in the
severity
of their corresponding phenotypes and the presence of ubiquitin-positive
protein aggregates suggesting that the deleterious effects associated
with
development of these aggregates might contribute to the complex
phenotypes observed in autophagy deficient mutants. Our results suggest
that Vmp1 is required for the clearance of these ubiquitinated protein
aggregates through autophagy and highlight a potential role for Vmp1 in
protein-aggregation diseases.
Submitted by Ricardo Escalante [[log in to unmask]]
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Transcription of the Dictyostelium discoideum mitochondrial genome
occurs from a single initiation site
Phuong Le, Paul Robert Fisher and Christian Barth
Department of Microbiology, La Trobe University, Kingsbury Drive,
Bundoora, Victoria 3086, Australia
RNA, in press
Transcription of the mitochondrial genome in Dictyostelium discoideum
gives
rise to eight major polycistronic RNA species that can be detected by
Northern
hybridization. In order to determine whether these transcripts could
possibly
derive from processing of even larger transcripts, Reverse Transcriptase
Polymerase Chain Reactions (RT-PCR) were performed in an attempt to
amplify the intervening regions between the eight major transcripts.
All but
one intervening regions were successfully reverse transcribed and
amplified,
indicating that even larger transcripts existed and that the eight major
transcripts detected previously may be the products of transcript
processing.
Southern hybridization analyses of DNA fragments representing the
sequences
between the eight major transcripts with in vitro capped mitochondrial
RNA
identified the 5' end of only one of the eight major transcripts as a
genuine
transcription start site. The ability to initiate transcription from
DNA sequences
upstream of the identified transcription initiation site was
demonstrated in
bacterial cells expressing the Dictyostelium mitochondrial RNA
polymerase.
We conclude that transcription of the Dictyostelium mitochondrial
genome is
initiated at a single site, generating a large polycistronic
transcript that is very
efficiently, probably co-transcriptionally, processed into mature RNA
species.
This is the first report on a protist mitochondrial DNA that is,
although much
larger in size than its metazoan counterparts, transcribed from a single
transcription initiation site.
Submitted by Christian Barth [[log in to unmask]]
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[End dictyNews, volume 33, number 14]
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