Flexible phenylalanine-glycine nucleoporins as entropic barriers to nucleocytoplasmic transport

doi:10.1073/pnas.0603521103

Flexible phenylalanine-glycine nucleoporins as entropic barriers to nucleocytoplasmic transport

*M. E. Müller Institute for Structural Biology, Biozentrum, University of Basel, Klingelbergstrasse 70, Basel 4056, Switzerland; and
^¶Institute of Materials Research and Engineering, 3 Research Link, Singapore 117602, Singapore

Communicated by Edwin W. Taylor, Northwestern University Feinberg School of Medicine, Chicago, IL, May 1, 2006
↵ ^‡N.-P.H. and J.K. contributed equally to this work. (received for review November 1, 2005)

Abstract

Natively unfolded phenylalanine-glycine (FG)-repeat domains are alleged to form the physical constituents of the selective barrier-gate in nuclear pore complexes during nucleocytoplasmic transport. Presently, the biophysical mechanism behind the selective gate remains speculative because of a lack of information regarding the nanomechanical properties of the FG domains. In this work, we have applied the atomic force microscope to measure the mechanical response of individual and clusters of FG molecules. Single-molecule force spectroscopy reveals that FG molecules are unfolded and highly flexible. To provide insight into the selective gating mechanism, an experimental platform has been constructed to study the collective behavior of surface-tethered FG molecules at the nanoscale. Measurements indicate that the collective behavior of such FG molecules gives rise to an exponentially decaying long-range steric repulsive force. This finding indicates that the molecules are thermally mobile in an extended polymer brush-like conformation. This assertion is confirmed by observing that the brush-like conformation undergoes a reversible collapse transition in less polar solvent conditions. These findings reveal how FG-repeat domains may simultaneously function as an entropic barrier and a selective trap in the near-field interaction zone of nuclear pore complexes; i.e., selective gate.

Footnotes

^†To whom correspondence should be addressed. E-mail: roderick.lim{at}unibas.ch
↵ ^§Present address: Concentris GmbH, P.O. Box 340, Davidsbodenstrasse 63, Basel 4012, Switzerland.
↵ ^‖Present address: Max Plank Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany.
Author contributions: R.Y.H.L., N.-P.H., J.K., J.D., and U.A. designed research; R.Y.H.L., N.-P.H., J.K., J.D., and K.H.A.L. performed research; K.S.-H. and B.F. contributed new reagents/analytic tools; R.Y.H.L. analyzed data; and R.Y.H.L. wrote the paper.
Conflict of interest statement: No conflicts declared.
Abbreviations:

Abbreviations:

AFM,

atomic force microscopy;

FG,

phenylalanine-glycine;

FV,

force–volume;

NLS,

nuclear localization signal;

NPC,

nuclear pore complex;

Nup,

nucleoporin;

SMFS,

single-molecule force spectroscopy.