Cell News | Issue 01, 2017 - page 32

Cell News 01/2017
Long coiled-coils as molecular motors:
entropic polymer engines
Marcus Jahnel (1, 2), David H. Murray (2), Marino Zerial (2), Stephan W. Grill (1, 2)
Presenting author: Marcus Jahnel
1 – BIOTEC, TU Dresden, Dresden, Germany,
2 – Max Planck Institute for Molecular Cell Biology and
Genetics, Dresden, Germany
Coiled-coils are ubiquitous protein motifs present in over 5%
of all eukaryotic proteins. Although these domains are often
deemed synonymous with stable and static structures, very
long coiled-coil regions (L > 30 nm) are surprisingly often
found in proteins that can generate and sense forces. Thus,
extended coiled-coil proteins take part in some of the most
intricate cellular processes: from vesicle tethering to the trans-
port of cargo, from the maintenance of chromosome structure
to the attachment of the kinetochore, from the motion of the
cytoskeleton to cytokinesis. Yet how can these slender mole-
cules achieve this?
Here we discuss our recent discovery [1] that the unusually
long membrane tethering protein early endosome antigen
1 (EEA1) – a 220 nm long coiled-coil – is rather rigid and
extended when only tethered at its base, yet becomes much
more flexible upon binding of the small GTPase Rab5 to its free
end. Importantly, using a reconstituted tethering system and
single-molecule optical tweezer experiments, we demonstrate
that this increased flexibility results in an entropic force that
can pull two objects together. Moreover, we show that GTP-hy-
drolysis acts as a timer for this interaction and that strategic
mutations to the coiled-coil can interfere with this effect.
Taken together, our experimental results suggest a thermody-
namic cycle in which the persistence length of a coiled-coil
biopolymer is modulated by ligand binding and NTP hydrolysis.
In this picture EEA1 and Rab5 work together as a two-com-
ponent molecular motor. We propose transient binding events
that switch slender molecules from rigid to flexible and back
as an effective mechanism to generate and sense forces on the
jiggling molecular level.
[1] An endosomal tether undergoes an entropic collapse to
bring vesicles together. Nature (2016).
Integrin beta 3 regulates actin cytoskeleton and
biomechanical properties of hematopoietic stem cells
cultured on bone marrow mimetic scaffolds
Martin Kraeter, Angela Jacobi, Oliver Otto, Stefanie Tietze, Katrin Mueller, David M. Poitz,
Sandra Palm, Valentina M. Zinna, Ulrike Biehain, Manja Wobus, Triantafyllos Chavakis,
Carsten Werner, Jochen Guck and Martin Bornhaeuser
Presenting author: Martin Kraeter
Medical Clinic I, Universital Hospital Carl Gustav Carus Dresden
The bone marrow (BM) microenvironment provides critical
physical cues for hematopoietic stem and progenitor cells
(HSPCs) and accommodates important cell-matrix interactions.
Due to a lack of suitable culture methods regarding particu-
larly the extracellular matrix (ECM) contact the mechanisms
underlying matrix communication and signal transduction are
less understood. We used a novel technology to mimic the BM
stroma by decellularized ECM scaffolds derived from mesen-
chymal stromal cells (MSCs). Using freshly isolated CD34+
HSPCs around 20 remain in the supernatant (SN-cells). The
same proportion of AT- and SN- cells could be observed on
control MSC monolayers. RGD-binding integrin (ITG) expres-
sion, namely integrin
IIb (CD41),
V (CD51) and
3 (CD61)
was found to be induced by ECM scaffold contact. Adhesion
leads to a highly migratory and polarized phenotype which is
accompanied with filamentous actin rearrangement. Accord-
ingly, probing biophysical properties we found AT-cells to have
the same compliance as freshly isolated BM HSPCs. In contrast,
SN-cells exhibit a more deformable phenotype. Inhibiting
3 signaling using a blocking antibody reduced migration
and polarization. This identifies focal contact formation via
3 as an important mediator of cell adhesion and migration
to BM stroma and highlight ECM-mediated cues as modulator
of HSPC biomechanical phenotype.
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