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Tactoidal Granules in Concentrated Actin Gels: A Solidlike State of Protein Filaments

Published online by Cambridge University Press:  15 February 2011

Jay. X. Tang ,
Rudolf Oldenbourg ,
Philip G. Allen  and
Paul A. Janmey
Jay. X. Tang
Affiliation:
Division of Experimental Medicine, Brigham & Women's Hospital, Boston, MA 02115, [email protected]
Rudolf Oldenbourg
Affiliation:
Marine Biological Laboratory, Woods Hole, MA 02543.
Philip G. Allen
Affiliation:
Division of Experimental Medicine, Brigham & Women's Hospital, Boston, MA 02115, [email protected]
Paul A. Janmey
Affiliation:
Division of Experimental Medicine, Brigham & Women's Hospital, Boston, MA 02115, [email protected]
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Abstract

We report here the first observation and microscopic characterization of tactoidal granules of actin in concentrated gels of actin filaments (F-actin). Phase contrast microscopy shows these stable tactoids of densely packed F-actin to be of various sizes on the order of 10 μm. The background gel of F-actin is optically birefringent, indicating orientational order of the filaments consistent with theoretical predictions. In contrast, no birefringence is detected through the tactoids, suggesting very distinct yet undetermined packing of the filaments inside. The tactoids demonstrate elastic response upon micromanipulation. The microscopic segregation of F-actin into two states of different protein concentrations is consistent with a first order phase transition between a nematic gel of lower concentration on the order of ten mg/nl and a highly packed, possibly columnar state of protein filaments. In addition to the excluded volume effects which are known to drive thermodynamic phase transitions as a function of particle concentration, additional molecular forces must also play important roles since the formation was found to be irreversible upon dilution.

Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 489: Symposium K – Materials Science of the Cell , 1997 , 33
Copyright
Copyright © Materials Research Society 1998

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