Freeze-fracture studies in brown algae: Putative cellulose-synthesizing complexes on the plasma membrane

Numerous single particles and particle complexes were observed on the protoplasmic fracture-face of the plasma membrane of the brown algae Ectocarpus siliculosus, Hincksia mitchelliae, Tilopteris mertensii, Sphacelaria rigidula, S. radicans and S. nana examined by freeze-fracture. The single particles do not show any particular distribution pattern, and their diameter varies from 7 nm to 10 nm. Two types of particle complex can be found: linear complexes, consisting of a single row of densely packed particles, and pentads of particles forming a rectangular configuration, with four particles in the corners and one in the centre. The number of particles in the linear complexes is between 6 and about 90. Their diameter is about 7.5 nm, which is smaller than that of most single particles. Although the linear complexes are not strictly oriented, two main axes can be distinguished which form an angle of about 45-90 degrees. This orientation appears to be parallel to that of the cellulose microfibrils. The linear complexes are frequently connected with the end of microfibril imprints (terminal complexes). The size of the particles of the pentads is similar to that of the single particles, and the side of the square is about 14 nm. The distribution and density of the pentads are variable. The pentads may be rare (e.g. in E. siliculosus), or there can be as many as 300 mu m(-2) (e.g. in T. mertensii). No such particle complexes were found on the extraplasmic fracture-face of the plasma membrane. The possible role of the above particle complexes is discussed.