Biology Reference
In-Depth Information
INTRODUCTION
Microtubules, a component of the cytoskeleton, are micrometer long, 25-nm wide
polymers of tubulin (
Lodish et al., 2007
). Microtubules are key players at various
stages in cellular life: cell division, cell movement, and intracellular transport. Each
of these functions requires microtubules which are stiff on cellular length scales—
that is, microtubules that bend relatively little over micrometer lengths. As a result,
the mechanical properties of microtubules have been an area of active research
for the past 2 decades (for a recent review, see
Hawkins, Mirigian, Selcuk Yasar, &
Ross, 2010
), because understanding the mechanics of individual microtubules contrib-
utes to modeling whole-cell rigidity and structure and hence to better understanding
the physical processes underlying motility and transport. Other work has explicitly
focused on the details of microtubule stiffness in the context of microtubule function:
the length-dependence of the mechanical properties of microtubules may govern their
role in whole-cell mechanics (
Mehrbod &Mofrad, 2011
); microtubule-associated pro-
teins (MAPs) can increase or decrease the flexibility of microtubules (
Cassimeris,
Gard, Tran,&Erickson, 2001; Felgner et al., 1997; Portranet al., 2013
); andmicrotubule
stiffness variationmay affectwhole-cellmorphology (
Topalidou et al., 2012
), to name a
few examples.
In addition, the intrinsic flexibility of microtubules shows substantial heteroge-
neity. Experimentally,
in vitro
measurements of mechanical rigidity (characterized
by the persistence length or, alternatively, the Young's modulus) of microtubules
have varied by an order of magnitude, with persistence lengths varying from 1 to
10 mm (
Brangwynne et al., 2007; Cassimeris et al., 2001; Gittes, Mickey,
Nettleton, & Howard, 1993; Janson & Dogterom, 2004; Valdman, Atzberger, Yu,
Kuei, & Valentine, 2012; van den Heuvel, Bolhuis, & Dekker, 2007; van
Mameren, Vermeulen, Gittes, & Schmidt, 2009
). Surprisingly, a number of experi-
ments have found that very short microtubules have substantially shorter persistence
lengths, on the scale of 0.02-0.1 mm (
Pampaloni et al., 2006; Taute, Pampaloni,
Frey, & Florin, 2008; Van den Heuvel, de Graaff, & Dekker, 2008
). And, experi-
ments on identical microtubules show mechanical rigidity variations, which are sub-
stantially broader than those expected from experimental imprecision alone
(
Valdman et al., 2012
). Theoretical models have been developed to describe these
effects (
Bathe, Heussinger, Claessens, Bausch, & Frey, 2008; Heussinger, Bathe, &
Frey, 2007; Pampaloni et al., 2006; Tounsi, Heireche, Benhassaini, & Missouri,
2010
), but these models differ in key predictions, predictions which still require
experimental examination.
As a result of this interest in microtubule mechanical properties, a number of
experimental techniques have been developed to measure microtubule flexibility;
in particular, active techniques which involve bending the polymer using optical
traps or electric fields (
Kikumoto, Kurachi, Tosa, & Tashiro, 2006; Van den
Heuvel et al., 2008
) and passive techniques which measure the fluctuations of free
polymers in solution (
Brangwynne et al., 2007; Gittes et al., 1993
). The active
measurements, however, require specialized setups to implement known forces on
