Available online at www.sciencedirect.com
Coordination of molecular motors: from in vitro assays to
intracellular dynamics
Erika LF Holzbaur and Yale E Goldman
New technologies have emerged that enable the tracking of
molecular motors and their cargos with very high resolution both
in vitro and in live cells. Classic in vitro motility assays are being
supplemented with assays of increasing complexity that more
closely model the cellular environment. In cells, the introduction
of probes such as quantum dots allows the high-resolution
tracking of both motors and vesicular cargos. The ‘bottom up’
enhancement of in vitro assays and the ‘top down’ analysis of
motility inside cells are likelyto converge over the next few years.
Together, these studies are providing new insights into the
coordination of motors during intracellular transport.
Address
Pennsylvania Muscle Institute and Department of Physiology, School of
Medicine, University of Pennsylvania, Philadelphia, PA 19104, United
States
Corresponding author: Holzbaur, Erika LF
(holzbaur@mail.med.upenn.edu) and Goldman, Yale E
(goldmany@mail.med.upenn.edu)
Current Opinion in Cell Biology 2010, 22:4–13
This review comes from a themed issue on
Cell structure and dynamics
Edited by Arshad Desai and Marileen Dogterom
Available online 25th January 2010
0955-0674/$ – see front matter
# 2010 Elsevier Ltd. All rights reserved.
DOI 10.1016/j.ceb.2009.12.014
Introduction
Molecular motors drive a myriad of essential processes in
the cell, including targeted delivery of vesicular cargos,
localization of organelles and mRNAs, and chromosome
and spindle dynamics during mitosis. The three classi-
cally described linear molecular motors, myosin, kinesin,
and dynein, carry their cargos along actin filaments (AFs)
and microtubules (MTs) in these transport roles, in
addition to their well-characterized functions in muscle
contraction and flagellar beating. New single molecule
fluorescence microscopy techniques and infrared optical
traps have led to remarkable progress in understanding
the transduction of metabolic energy into mechanical
force and motion, using in vitro systems combining pur-
ified motors with their tracks on glass microscope slides
(Box 1). Of course, transport in the cell occurs in an
environment that is considerably more complex than is
reflected in these simplified assays of motor function.
Intersecting and bundled networks of cytoskeletal fila-
ments, obstacles such as actin-binding or microtubule-
associated proteins bound along motor tracks, and the
coordination of multiple motor types bound to the same
cargos are all likely to affect motility within the cell
(Figure 1). The gap between simplified in vitro assays
of motor activity and the biology of intracellular transport
in vivo is being bridged from both sides. Here we consider
‘bottom up’ experimental approaches using purified com-
ponents in motility assays modified to reflect elements of
complexity present within cells, as well as ‘top down’
studies of motors tagged with fluorescent reporters such
as quantum dots (QDs) and introduced into cells. The
results of these experimental studies on the coordination
and collective properties of molecular motors can be
compared to theoretical predictions, reviewed in this
volume by Gue
´
rin et al. [1].
Processivity and gating
The three major motor families, kinesins, dyneins, and
myosins, show considerable variation in structure, speed,
and number of steps taken per diffusional encounter with
their cytoskeletal track (mechanical processivity). The
paradigm is a highly processive, two-headed motor, such
as kinesin-1, which drives cargos toward the plus end of
microtubules. Cytoplasmic dynein is also a highly pro-
cessive two-headed motor, but moves toward MT minus
ends. Similarly, myosin-V and myosin-VI move in oppo-
site directions along actin filaments. Kinesin and dynein
produce repeated steps along microtubules at the 8 nm
periodicity of the tubulin dimers, although dynein can
exhibit larger steps as well as reversals [2,3]. Myosin-V
takes 36 nm steps toward the barbed end of actin, corre-
sponding approximately to the half-pitch of actin’s double
helix; whereas myosin-VI exhibits pointed end-directed
steps of 20–40 nm [4]. Each of these examples has been
shown to move with a ‘hand-over-hand’ gait [5–8]in
which both heads can bind simultaneously to their track.
Their stride involves detachment and forward motion of
the trailing head to become the leader (Figure 2a ! b).
High processivity implies a mechanism to prevent both
heads from dissociating from the track at the same time.
For kinesin and myosin-V, thermal fluctuations stretch
the two heads apart to reach the next filament-binding
site at the completion of the step, leading to an intramo-
lecular force that pulls the leading head backward and the
trailing head forward (colored arrows in Figure 2a and b).
The intramolecular forces on the two heads are the most
likely signals that cause the trailing head to detach while
Current Opinion in Cell Biology 2010, 22:4–13 www.sciencedirect.com