dictyNews
Electronic Edition
Volume 37, number 8
September 30, 2011

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Abstracts
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Dictyostelium Chemotaxis: Essential Ras activation and accessory 
signaling pathways for amplification

Arjan Kortholt, Rama Kataria, Ineke Keizer-Gunnink, Wouter N. 
Van Egmond, Ankita Khanna and Peter J.M. Van Haastert

Department of Cell Biochemistry, University of Groningen, Nijenborgh 7, 
9747 AG Groningen, The Netherlands


EMBO reports, in press

Central to chemotaxis is the molecular mechanism by which cells exhibit 
directed movement in shallow gradients of chemoattractant. We used 
Dictyostelium mutants to investigate the minimal requirements for chemotaxis, 
and identified a basal signaling module providing activation of Ras at the 
leading edge, which is sufficient for chemotaxis. The signaling enzymes PI3K, 
TorC2, PLA2 and sGC are not required for Ras activation and chemotaxis to 
folate or to steep gradients of cAMP, but they provide a memory of direction 
and improved orientation of the cell, which together increases the sensitivity 
~150-fold for chemotaxis in shallow cAMP gradients.


Submitted by: Peter van Haastert [[log in to unmask]]
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Stretching actin filaments within cells enhances their affinity for the 
myosin ii motor domain.

Taro Q.P. Uyeda1,2, Yoshiaki Iwadate3, 4, Nobuhisa Umeki1, Akira 
Nagasaki1 and Shigehiko Yumura3 

1: Biomedical Research Institute, National Institute of Advanced Industrial 
Science and Technology (AIST), Tsukuba, Ibaraki, Japan
2: Biomedicinal Information Research Center, National Institute of Advanced 
Industrial Science and Technology (AIST), Koto, Tokyo, Japan
3: Department of Functional Molecular Biology, Graduate School of Medicine, 
Yamaguchi University, Yamaguchi, Yamaguchi, Japan
4: Precursory Research for Embryonic Science and Technology (PRESTO), 
Japan Science and Technology Agency (JST), Kawaguchi, Saitama, Japan


PLoS One, in press

To test the hypothesis that the myosin II motor domain (S1) preferentially 
binds to specific subsets of actin filaments in vivo, we expressed GFP-fused 
S1 with mutations that enhanced its affinity for actin in Dictyostelium cells.  
Consistent with the hypothesis, the GFP-S1 mutants were localized along 
specific portions of the cell cortex.  Comparison with rhodamine-phalloidin 
staining in fixed cells demonstrated that the GFP-S1 probes preferentially 
bound to actin filaments in the rear cortex and cleavage furrows, where actin 
filaments are stretched by interaction with endogenous myosin II filaments.  
The GFP-S1 probes were similarly enriched in the cortex stretched passively 
by traction forces in the absence of myosin II or by external forces using a 
microcapillary.  The preferential binding of GFP-S1 mutants to stretched 
actin filaments did not depend on cortexillin I or PTEN, two proteins 
previously implicated in the recruitment of myosin II filaments to stretched 
cortex.  These results suggested that it is the stretching of the actin filaments 
itself that increases their affinity for the myosin II motor domain.  In contrast, 
the GFP-fused myosin I motor domain did not localize to stretched actin 
filaments, which suggests different preferences of the motor domains for 
different structures of actin filaments play a role in distinct intracellular 
localizations of myosin I and II.  We propose a scheme in which the 
stretching of actin filaments, the preferential binding of myosin II filaments 
to stretched actin filaments, and myosin II-dependent contraction form a 
positive feedback loop that contributes to the stabilization of cell polarity 
and to the responsiveness of the cells to external mechanical stimuli.  


Submitted by: Taro Uyeda [[log in to unmask]]
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[End dictyNews, volume 37, number 8]