THE FORMATION AND DEVELOPMENT OF CELLULOSE-SYNTHESIZING LINEAR TERMINAL COMPLEXES (TCS) IN THE PLASMA-MEMBRANE OF THE MARINE RED ALGA ERYTHROCLADIA-SUBINTEGRA ROSENV

The formation and development of linear terminal complexes (TCs), the putative cellulose synthesizing units of the red alga Erythrocladia su bintegra Rosenv., were investigated by a freeze etching technique usin g both rotary and unidirectional shadowing. The ribbon-like cellulose fibrils of E. subintegra are 27.6+/-0.8 nm wide and only 1-1.5 nm thic k. They are synthesized by TCs which are composed of repeating transve rse rows formed of four particles, the TC subunits. About 50.4 +/- 1.7 subunits constitute a TC, They are apparently more strongly interconn ected in transverse than in longitudinal directions. Some TC subunits can be resolved as doublets by Fourier analysis. Large globular partic les (globules) seem to function as precursor units in the assembly and maturation of the TCs. They are composed of a central hole (the core) with small subunits forming a peripheral ridge and seem to represent zymogenic precursors. TC assembly is initiated after two or three gobu les come into close contact with each other, swell and unfold to a nuc leation unit resembling the first 2-3 transverse rows of a TC. Longitu dinal elongation of the TC occurs by the unfolding of globules attache d to both ends of the TC nucleation unit until the TC is completed. Th e typical intramembranous particles observed in Erythrocladia (unidire ctional shadowing) are 9.15 +/- 0.13 nm in diameter, whereas those of a TC have an average diameter of 8.77 +/- 0.11 nm. During cell wall sy nthesis membranes of vesicles originating from the Golgi apparatus and which seem to fuse with the plasma membrane contain large globules, 1 5-22 nm in diameter, as well as ''tetrads'' with a particle diameter o f about 8 nm. The latter are assumed to be involved in the synthesis o f the amorphous extracellular matrix cell wall polysaccharides. The fo llowing working model for cellulose fibril assembly in E. subintegra i s suggested: (1) the ribbon-like cellulose fibril is syn thesized by a single linear TC; (2) the number of glucan chains per microfibril cor relates with the number of TC subunits; (3) a single subunit synthesiz es 3 glucan chains which appear to stack along the 0.6 nm lattice plan e; (4) lateral aggregation of the ''3-mer'' stacks leads to the crysta lline microfibril.