dictyNews
Electronic Edition
Volume 33, number 15
December 4, 2009

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


Tug-of-war between Dissimilar Teams of Microtubule Motors Regulates
Transport and Fission of Endosomes

Soppina V, Rai AK, Ramaiya AJ, Barak P, Mallik R.


Proceedings of the National Academy of Sciences,106, 19381 (2009)

Intracellular transport is interspersed with frequent reversals in  
direction
due to the presence of opposing kinesin and dynein motors on organelles
that are carried as cargo. The cause and the mechanism of reversals are
unknown, but are a key to understanding how cargos are delivered in a
regulated manner to specific cellular locations. Unlike established
single-motor biophysical assays,this problem requires understanding of
the cooperative behavior of multiple interacting motors.

Here we present measurements inside live Dictyostelium cells, in a cell
extract and with purified motors to quantify such an ensemble function  
of
motors. We show through precise motion analysis that reversals during
endosome motion are caused by a tug-of-war between kinesin and dynein.
Further, we use a combination of optical trap-based force measurements
and Monte Carlo simulations to make the surprising discovery that
endosome transport uses many (approximately four to eight) weak and
detachment-prone dyneins in a tug-of-war against a single strong and
tenacious kinesin. We elucidate how this clever choice of dissimilar
motors and motor teams achieves net transport together with endosome
fission, both of which are important in controlling the balance of  
endocytic
sorting. To the best of our knowledge, this is a unique demonstration  
that
dynein and kinesin function differently at the molecular level inside  
cells
and of how this difference is used in a specific cellular process,  
namely
endosome biogenesis. Our work may provide a platform to understand
intracellular transport of a variety of organelles in terms of  
measurable
quantities.


Submitted by Roop Mallik [[log in to unmask]]
--------------------------------------------------------------------------------


Myosin II Is Essential for the Spatiotemporal Organization of Traction
Forces during Cell Motility

Ruedi Meili(1),* Baldomero Alonso-Latorre (1)+, Juan C. del Álamo+,
Richard A. Firtel(2),* and Juan C. Lasheras(2)+

*Section of Cell and Developmental Biology, Division of Biological  
Sciences,
+Department of Mechanical and Aerospace Engineering, and ||Department
of Bioengineering, University of California, San Diego, La Jolla, CA  
92093
1- co-first authors
2- co-senior authors


Mol Bio Cell, in press

Amoeboid motility requires spatiotemporal coordination of biochemical
pathways regulating force generation and consists of the quasi-periodic
repetition of a motility cycle driven by actin polymerization and  
actomyosin
contraction. Using new analytical tools and statistical methods, we  
provide,
for the first time, a statistically-significant quantification of the  
spatial
distribution of the traction forces generated at each phase of the cycle
(protrusion-contraction-retraction-relaxation). We show that cells are
constantly under tensional stress and that wild-type cells develop two
opposing "pole" forces pulling the front and back toward the center  
whose
strength is modulated up and down periodically in each cycle. We
demonstrate that MyoII cross-linking and motor functions have different
roles in controlling the spatiotemporal distribution of traction  
forces, the
changes in cell shape, and the duration of all the phases. We show that
the time required to complete each phase is dramatically increased in
cells with altered MyoII-motor-function, demonstrating that it is  
required
not only for contraction but also for protrusion. Concomitant loss of
MyoII-actin-cross-linking leads to a force redistribution throughout  
the cell
perimeter pulling inwards toward the center. However, it does not reduce
significantly the magnitude of the traction forces, uncovering a
nonMyoII-mediated mechanism for the contractility of the cell.


Submitted by Rick Firtel [[log in to unmask]]
--------------------------------------------------------------------------------


The WD-Repeat Domain of Dictyostelium Myosin Heavy Chain Kinase C
Functions in Both Substrate Targeting and Cellular Localization

Atiya Franklin, Linzi Hyatt, Alyssa Chowdhury, Paul A. Steimle


J Eukaryotic Cell, in press

In nonmuscle cells, contraction-dependent processes such as cytokinesis
and cell migration rely on the proper assembly and localization of  
myosin II
bipolar filaments; however, the signalling pathways regulating these
processes are poorly understood.  Studies in Dictyostelium discoideum,
and recently in mammalian cells, have demonstrated that myosin II
filament disassembly is driven by phosphorylation of the tail region of
the myosin II heavy chain (MHC), preventing myosin-mediated contraction.
In Dictyostelium, MHC phosphorylation is catalyzed by three MHC kinases
(MHCK-A, -B, and -C) that share homologous alpha-kinase and WD-repeat
domains. As a means of understanding how each of the MHCKs functions
in the cell, we have focused our studies on the structure-function
relationships defining the activity of MHCK-C.  Our cellular and  
biochemical
studies revealed that the WD-repeat domain of MHCK-C is required for
targeting of the kinase to phosphorylate MHC.  Localization studies
demonstrated that MHCK-C translocation to the cell cortex occurs in the
presence of chemoattractant, with a timing that lags behind myosin II
cortical enrichment.  We also show that the MHCK-C WD-repeat domain
is unique among its MHCK-A and MHCK-B counterparts since it is both
necessary and sufficient for proper localization of MHCK-C in the cell.
The posterior localization of this domain in chemotaxing cells occurs  
with
a periodicity similar to that exhibited by myosin II.  The unique  
functions
that we have identified for the MHCK-C WD-repeat domain allow for the
possibility that the seemingly redundant MHCKs in Dictyostelium could
be regulated by a single signal but to perform distinct roles in  
regulating
myosin II activity in the cell.


Submitted by Paul Steimle [[log in to unmask]]
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[End dictyNews, volume 33, number 15]