1. Contribution from the Department of Biomaterials Science, Graduate School of Agricultural and Life Sciences, The University of Tokyo, Tokyo 113-8657, Japan, Biosciences Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, and Centre de Recherches sur les Macromolécules VégétalesCNRS, affiliated with the Joseph Fourier University of Grenoble, B.P. 53, 38041 Grenoble Cedex 9, France