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KINKLIKE EXCITATIONS AS AN ENERGY TRANSFER

MECHANISM IN MICROTUBLES

M.V. Sataric(l), J.A. Tuszynski(2) and R.B. Zakula(3)

1) Faculty of Technical Sciences, 2100 Novi Sad, F .R. Yugoslavia

2) Department of Physics, Universityof Alberta, Edmonton, Canada, T6G 2Jl

3) Institute of N uclear Sciences "Boris Kidric" , Belgrade, F .R. Yugoslavia

~29-C). Twoof the Uni-

::know/edgesshop. ABSTRACT

A model is presented which is intended to provide a realistic physical picture ofthe energy transfer mechanism in cell microtubles (MT). A classical 4>4-model in thepresence of a constant electric field is used as a conceptual basis. It is demonstrated thatif kink-like excitations arise as a result of GTP hydrolysis then an intrinsic electricalforce may cause them to propagate along the microtuble. A discussion is given on thepossible effects on these excitations on the dynamics of microtubles.

Systems",

93:1502 INTRODUCTION;~~,

~MT's represent hono~cylinders 25 nanometers in diameter formed bypr9to~la-

ments aligned along their axes and whose length L may span macroscopic dimensions.In vivo, the cylindrical walls of MT's are assemblies of 13 longitudinal protofilaments,each of which is a series of subunit proteins known as tubulin dimmmers. (Fig. 1) Eachtubulin subunit is a polar, 8-nfi dimmer which consists of two, slightly different 4-nfi aand {3 monomers with molecular weight of 55 kilodaltons. Each dimmmer may be physi-cally viewed as an electric dipole whose dipolar character originates from 18 calciumions bound within a monomer. Thus MT's can be identified as an example of elec-tret substances, ie. oriented assemblies of dipoles. Barnett(l) conjectured that MT'sare processing channels along which strings of information bits can move transferringmessages from place to place. In this paper we examine a theoretical model based onthe creation of link-like excitations in MT's. Their presence will be linked to GTP

hydrolysis and the energy released therein.

Systems",

119Nonlinear Coherent Structures in Physics and BiologyEdited by K.H. Spatschek and F.G. Menens, Plenum Press, New York, 1994

ANALY~

IN A PE:

While the model presented here is closely related to the one developed by- Collins etal. (4) for ferroelectrics, we should mention that the effects of discreteness of the latticemay play an important role. It appears that fast-propagating kinks are more stablethan slow ones. The numerical simulations confirm the conclusion that for all noisestrengths studied, slow kinks are more affected by the perturbation than fast ones. Wehave seen in our model that the intrinsic electric field governs the rate of propagationof KLE's. By adding an external electric field parallel to MT we can introduce a newcontrol mechanism in the MT dynamics. An applied electric field will result in a fastermoving population of KLE's and thus a greater stability against thermal fluctuations.If, on the other hand, the intrinsic and applied electric fields are oriented in oppositedirections, then the KLE's motion may be slowed down ,or stopped altogether. Thiscan be seen as a basis for treating MT's as artificial information strings. Each KLEwithin a MT can be viewed as a bit of information whose propagation can be controlled

by an external electric field.An important experimental fact is that stability of MT'sis greatly enhanced by the

presence of lateral cross-bridging proteins, microtubleassociated proteins (MAP's).From the physical point of view, these bridges represent lattice impurities in the MTstructure. Interactions between soliton-like pulses and impurities have been very ex-tensively studied in the past few years. It was-demonstrated that impurity modes playan important role in soliton propagation. In,.p;articulaJ;1'~in~s~aybe totally reflectedby an attractive impurity if their velocities lie in specific);es9nan~e velocity 'windows'.Hence, if the velocity of KLE's lies within such a 'window', the rate of arrival of KLE'sat the ,-, end will decrease or even stop, as the number of MAP's increases. This me-chanism can lead to a significantly reduced rate of MT disassembly. This mechanismtogether with the control mechanism offered by an exteinal'electricfield may providea number of interesting possibilities for information transmission and storage in MT's.

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REFERENCES

1. M.P. Barnett, in F. Carter (ed.), Proceedings of the Third Molecular ElectronicDevice Conference (Naval Research Laboratory, Washington, D.C. 1987)

2. R. Melki, M.F. Carlier, D. Pantaloni and S.N. Timashett, Biochem. 28, 9143

(1989)

E48, 589 (1993).A. Tuszyllski and R.B. Zakula, Phys. Re3. M. V. Sa.taric

Ross, Phys. Rev. B19, 3630 (1979)4. M.A. Collins, A. Blumen, J.F. Currie and

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