Nematic liquid crystalline formation of F-actin displays features of a continuous transition
- PDF / 1,110,934 Bytes
- 10 Pages / 612 x 792 pts (letter) Page_size
- 46 Downloads / 175 Views
Nematic liquid crystalline formation of F-actin displays features of a continuous transition Jorge Viamontes and Jay X. Tang* Physics Department, Indiana University, 727 East Third St Bloomington, IN 47405, U.S.A. *E-mail: [email protected] ABSTRACT The phase transition of solutions of the protein filaments F-actin from the isotropic (I) to the nematic (N) liquid crystalline state was studied by quantitative measurements of optical birefringence and fluorescence. The threshold protein concentration for the transition varies inversely with the average filament length, consistent with the prediction of statistical mechanics of rodlike suspensions based on the excluded volume effect. By measurements of local optical birefringence, a range of actin concentration is identified as the transition region between the isotropic and nematic phases. However, local measurements of the protein concentrations detect no discontinuity within a large number of samples in the transition region, suggesting that the I-N transition for F-actin occurs continuously over a defined concentration range. Thus the I-N transition appears to be of a higher order than the 1st, for F-actin of average filament length 3 µm or longer. Additionally, by mixing a tiny number of labeled actin filaments with the unlabeled ones, we observed thermal motions of individual filaments, thus ruling out an alternative picture, viewing an F-actin solution as an amorphous gel in which long filaments are too entangled or even cross-linked to allow partitioning of filaments into domains of discontinuous concentrations. We propose based on these experimental findings that either the extreme polydispersity of F-actin, or the large average filament length itself, renders the I-N transition to be a continuous one, in terms of both the average alignment of filaments and the protein concentration. INTRODUCTION Actin was discovered in 1943 by F.B Straub as one of the major proteins of the muscle tissue [1]. Nearly identical isoforms of the same protein were later found abundant in the cytoplasm of virtually all eukaryotic cells. Actin plays an important role in maintenance of cell shape, cell motility, and cytokinesis [2,3]. Central to many functions of actin is its polymerization property. The 42 kd monomers can rapidly self-assemble to form long filaments, called F-actin. Actin filaments in the cytoplasm are found in various states: isotropic network, liquid crystalline domains, paracrystalline bundles, etc. [4]. Understanding this whole range of structures, as well as many dynamic aspects of actin assembly, forms the physical basis of research on cell motility. In vitro, F-actin forms a nematic liquid crystalline phase at slightly above 2 mg/ml protein concentration [5-8]. The formation of such a liquid crystalline phase for a suspension of rodlike molecules is a common phenomenon experimentally, and was first predicted theoretically over 50 years ago [9]. Several recent studies have confirmed many aspects of the theoretical predictions for the case of F-actin solutions.
Data Loading...
