dictyNews Volume 38 Number 14

eZine lover (@eZine)

Published in 

Dicty News

 · 11 months ago

dictyNews 
Electronic Edition 
Volume 38, number 14 
May 25, 2012 

Please submit abstracts of your papers as soon as they have been 
accepted for publication by sending them to dicty@northwestern.edu 
or by using the form at 
http://dictybase.org/db/cgi-bin/dictyBase/abstract_submit. 

Back issues of dictyNews, the Dicty Reference database and other 
useful information is available at dictyBase - http://dictybase.org. 

Follow dictyBase on twitter: 
http://twitter.com/dictybase 

 
========= 
Abstracts 
========= 

 
Unusual combinatorial involvement of poly-A/T tracts in organizing  
genes and chromatin in Dictyostelium 

 
Gue Su Chang1, Angelika A Noegel3, Travis N Mavrich1, Rolf MŸller3,  
Lynn Tomsho1, Elissa Ward1, Marius Felder4, Cizhong Jiang1,6,  
Ludwig Eichinger5, Gernot Glšckner2, Stephan Schuster1,  
B. Franklin Pugh1# 

 
Genome Res. 2012, published online before print. 

Dictyostelium discoideum is an amoebozoa that exists in both a free-living  
unicellular and a multi-cellular form. It is situated in a deep branch in the  
evolutionary tree, and is particularly noteworthy in having a very A/T-rich  
genome.  Dictyostelium provides an ideal system to examine the extreme  
to which nucleotide bias may be employed in organizing promoters, genes,  
and nucleosomes across a genome.  We find that Dictyostelium genes are  
demarcated precisely at their 5Õ ends by poly-T tracts and precisely at their  
3Õ ends by poly-A tracts.  These tracts are also associated with  
nucleosome-free regions, and are embedded with precisely positioned TATA  
boxes.  Homo- and heteropolymeric tracts of A and T demarcate nucleosome  
border regions.  Together these findings reveal the presence of a variety of  
functionally distinct polymeric A/T elements.  Strikingly, Dictyostelium  
chromatin may be organized in di-nucleosome units, but is otherwise  
organized as in animals. This includes a +1 nucleosome in a position that  
predicts the presence of a paused RNA polymerase II.  Indeed, we find a  
strong phylogenetic relationship between the presence of the NELF pausing  
factor and positioning of the +1 nucleosome.  Pausing and +1 nucleosome  
positioning may have co-evolved in animals. 

 
Submitted by Gue Su Chang [gxc187@psu.edu] 
-------------------------------------------------------------------------------------- 

 
AprA functions as an endogenous chemorepellant in Dictyostelium discoideum 

Jonathan E. Phillips and Richard H. Gomer 

 
PNAS, in press 

Chemorepellants may play multiple roles in physiological and pathological  
processes. However, few endogenous chemorepellants have been  
identified, and how they function is unclear. We found that the autocrine  
signal AprA, which is produced by growing Dictyostelium discoideum cells  
and inhibits their proliferation, also functions as a chemorepellant. Wild-type  
cells at the edge of a colony show directed movement outwards from the  
colony, whereas cells lacking AprA do not. Cells show directed movement  
away from a source of recombinant AprA and dialyzed conditioned media  
from wild-type cells, but not dialyzed conditioned media from aprAø cells.  
The secreted protein CfaD, the G protein Gα8, and the kinase QkgA  
are necessary for the chemorepellant activity of AprA as well as its  
proliferation-inhibiting activity, whereas the putative transcription factor  
BzpN is dispensable for the chemorepellant activity of AprA but necessary  
for inhibition of proliferation. Phospholipase C and PI3-kinases 1 and 2,  
which are necessary for the activity of at least one other chemorepellant in  
Dictyostelium, are not necessary for rAprA chemorepellant activity. Starved  
cells are not repelled by rAprA, suggesting that aggregation-phase cells are  
not sensitive to the chemorepellant effect.  Cell tracking indicates that AprA  
affects the directional bias of cell movement, but not cell velocity or the  
persistence of cell movement. Together, our data indicate that the  
endogenous signal AprA acts as an autocrine chemorepellant for  
Dictyostelium cells. 

 
Submitted by Richard Gomer [rgomer@mail.bio.tamu.edu] 
-------------------------------------------------------------------------------------- 

 
CP55, a novel key component of centrosomal organization in Dictyostelium 

Oliver Kuhnert, Otto Baumann, Irene Meyer and Ralph GrŠf 

University of Potsdam, Dept. of Cell Biology, Potsdam-Golm, Germany 

 
Cell. Mol. Life Sci. in press 

Dictyostelium centrosomes consist of a layered core structure surrounded  
by a microtubule-nucleating corona. At the G2/M transition the corona  
dissociates, and the core structure duplicates yielding two spindle pole  
bodies. Finally, in telophase the spindle poles mature into two new,  
complete centrosomes. CP55 was identified in a centrosomal proteome  
analysis. It is a component of the centrosomal core structure, and persists 
at the centrosome throughout the entire cell cycle. FRAP experiments  
revealed that during interphase the majority of centrosomal GFP-CP55 is  
immobile, which indicates a structural task of CP55 at the centrosome.  
The CP55 null mutant is characterized by increased ploidy, a less  
structured, slightly enlarged corona, and by supernumerary, cytosolic  
MTOCs, containing only corona proteins and lacking a core structure.  
Live cell imaging showed that supernumerary MTOCs arise in telophase. 
Lack of CP55 also caused premature recruitment of the corona organizer  
CP148 to mitotic spindle poles, already in metaphase instead of telophase.  
Forces transmitted through astral microtubules may expel prematurely  
acquired or loosely attached corona fragments into the cytosol, where they  
act as independent MTOCs. CP55null cells were also impaired in growth, 
most probably due to difficulties in centrosome splitting during prophase.  
Furthermore, although they were still capable of phagocytosis, they  
appeared unable to utilize phagocytosed nutrients. This inability may be  
attributed to their partially disorganized Golgi apparatus. 

 
Submitted by Ralph GrŠf [rgraef@uni-potsdam.de] 
-------------------------------------------------------------------------------------- 

 
Identification of the kinase that activates a non-metazoan STAT gives  
insights into the evolution of phosphotyrosine-SH2 domain signaling  

Tsuyoshi Araki1, Takefumi Kawata2  and Jeffrey G. Williams+1 

1 College of Life Sciences, Welcome Trust Biocentre, University of Dundee, 
Dow St., Dundee, DD1 5EH, UK 
2 Department of Biology,  Faculty of Science, Toho University, 2-2-1 Miyama,  
Funabashi, Chiba 274-8510, Japan 

 
PNAS in press 

SH2 domains are integral to many animal signaling pathways. By  
interacting with specific phosphotyrosine residues, they provide regulatable  
protein-protein interaction domains. Dictyostelium is the only non-metazoan  
with functionally characterised SH2 domains but the cognate tyrosine  
kinases are unknown.  There are no orthologues of the animal tyrosine  
kinases but there are very many tyrosine kinase-like kinases (TKLs); a group  
of kinases which, despite their family name, are mainly classified as  
serine-threonine kinases. STATs are transcription factors that dimerise via  
phosphotyrosine-SH2 domain interactions. STATc is activated, by  
phosphorylation on Tyr922, when cells are exposed to the prestalk inducer  
DIF-1: a chlorinated hexaphenone. We show that in a null mutant for Pyk2,  
a tyrosine-specific TKL, exposure to DIF-1 does not activate STATc.  
Conversely, over-expression of Pyk2 causes constitutive STATc activation.  
Pyk2 phosphorylates STATc on Tyr922 in vitro and complexes with STATc  
both in vitro and in vivo. This demonstration that a TKL directly activates a  
STAT has significant implications for understanding the evolutionary origins  
of SH2 domain-phosphotyrosine signaling. It also has mechanistic  
implications. Our previous work suggested that a predictedly constitutive  
STATc tyrosine kinase activity is counterbalanced in vivo by the DIF-1  
regulated activity of PTP3: a Tyr922 phosphatase. Here we show that the  
STATc-Pyk2 complex is formed constitutively, by an interaction between  
the STATc SH2 domain and phosphotyrosine residues on Pyk2 that are  
generated by autophosphorylation. Also Pyk2 is, as predicted, constitutively  
active as a STATc kinase.  This observation provides further evidence for  
this highly atypical, possibly ancestral, STAT regulation mechanism.  

 
Submitted by Jeff Williams [j.g.williams@dundee.ac.uk] 
-------------------------------------------------------------------------------------- 

 
An orthologue of the Myelin-gene Regulatory Transcription Factor  
regulates Dictyostelium prestalk differentiation  

Hiroshi Senoo, Hong Yu Wang, Tsuyoshi Araki, Jeffrey G. Williams  
and Masashi Fukuzawa 

 
Int. J. of Dev. Biol., in press 

Background The prestalk region of the Dictyostelium slug is comprised  
of an anterior population of pstA cells and a posterior population of pstO  
cells. They are distinguished by their ability to utilize different parts of the  
promoter of the ecmA gene. Methods We identify, by mutational analysis  
and DNA transformation, CA-rich sequence elements within the ecmA  
promoter that are essential for pstA-specific expression and sufficient to  
direct pstA-specific expression when multimerised. The CA-rich region  
was used in affinity chromatography with nuclear extracts and bound  
proteins were identified by mass spectrometry. Results The CA-rich  
elements purify MrfA, a protein with extensive sequence similarity to  
animal Myelin-gene Regulatory Factor (MRF)-like proteins. The MRF-like  
proteins and MrfA also display more spatially limited but significant  
sequence similarity with the DNA binding domain of the yeast Ndt80  
sporulation-specific transcription factor. Furthermore, the ecmA CA- rich  
elements show sequence similarity to the core consensus Ndt80 binding  
site (the MSE) and point mutation of highly conserved arginine residues  
in MrfA, that in Ndt80 make critical contacts with the MSE, ablate binding  
of MrfA to its sites within the ecmA promoter. MrfA null strains are delayed  
in multicellular development and highly defective in pstA-specific gene  
expression. Conclusions These results provide a first insight into the  
intracellular signaling pathway that directs pstA differentiation and identify  
a non-metazoan orthologue of a family of molecularly uncharacterised  
transcription factors. 

 
Submitted by Masashi Fukuzawa [fukuzawa@cc.hirosaki-u.ac.jp] 
============================================================== 
[End dictyNews, volume 38, number 14]