1. Biosciences Division, M888, Los Alamos National Laboratory, Los Alamos, New Mexico 87545; Department of Biomaterials Science, Graduate School of Agricultural and Life Sciences, The University of Tokyo, Tokyo 113-8657, Japan; and Centre de Recherches sur les Macromolécules Végétales-CNRS, B.P. 53, 38041 Grenoble Cedex 9, France