1: Nature  1999 Jun 3;399(6735):446-8 

Tying a molecular knot with optical tweezers.

Arai Y, Yasuda R, Akashi K, Harada Y, Miyata H, Kinosita K Jr, Itoh H.

Department of Physics, Faculty of Science and Technology, Keio University,
Yokohama, Japan.

Filamentous structures are abundant in cells. Relatively rigid filaments, such
as microtubules and actin, serve as intracellular scaffolds that support
movement and force, and their mechanical properties are crucial to their
function in the cell. Some aspects of the behaviour of DNA, meanwhile, depend
critically on its flexibility-for example, DNA-binding proteins can induce sharp
bends in the helix. The mechanical characterization of such filaments has
generally been conducted without controlling the filament shape, by the
observation of thermal motions or of the response to external forces or flows.
Controlled buckling of a microtubule has been reported, but the analysis of the
buckled shape was complicated. Here we report the continuous control of the
radius of curvature of a molecular strand by tying a knot in it, using optical
tweezers to manipulate the strand's ends. We find that actin filaments break at
the knot when the knot diameter falls below 0.4 microm. The pulling force at
breakage is around 1 pN, two orders of magnitude smaller than the tensile stress
of a straight filament. The flexural rigidity of the filament remained unchanged
down to this diameter. We have also knotted a single DNA molecule, opening up
the possibility of studying curvature-dependent interactions with associated
proteins. We find that the knotted DNA is stronger than actin.

PMID: 10365955 [PubMed - indexed for MEDLINE]