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  • 1
    ISSN: 1615-6102
    Keywords: Erythrocladia subintegra ; Red alga ; Freeze-fracture ; Cellulose ; Linear terminal complexes
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Summary The formation and development of linear terminal complexes (TCs), the putative cellulose synthesizing units of the red algaErythrocladia subintegra Rosenv., were investigated by a freeze etching technique using both rotary and unidirectional shadowing. The ribbon-like cellulose fibrils ofE. subintegra are 27.6 ± 0.8 nm wide and only 1–1.5 nm thick. They are synthesized by TCs which are composed of repeating transverse rows formed of four particles, the TC subunits. About 50.4 ± 1.7 subunits constitute a TC. They are apparently more strongly interconnected in transverse than in longitudinal directions. Some TC subunits can be resolved as doublets by Fourier analysis. Large globular particles (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 gobules come into close contact with each other, swell and unfold to a nucleation unit resembling the first 2–3 transverse rows of a TC. Longitudinal elongation of the TC occurs by the unfolding of globules attached to both ends of the TC nucleation unit until the TC is completed. The typical intramembranous particles observed inErythrocladia (unidirectional 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 synthesis membranes of vesicles originating from the Golgi apparatus and which seem to fuse with the plasma membrane contain large globules, 15–22 nm in diameter, as well as ‘tetrads” with a particle diameter of about 8 nm. The latter are assumed to be involved in the synthesis of the amorphous extracellular matrix cell wall polysaccharides. The following working model for cellulose fibril assembly inE. subintegra is suggested: (1) the ribbon-like cellulose fibril is synthesized by a single linear TC; (2) the number of glucan chains per microfibril correlates with the number of TC subunits; (3) a single subunit synthesizes 3 glucan chains which appear to stack along the 0.6 nm lattice plane; (4) lateral aggregation of the “3-mer” stacks leads to the crystalline microfibril.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1615-6102
    Keywords: Erythrocladia subintegra ; Cellulose microfibrils ; Cellulosesynthesizing terminal complexes ; Terminal complex subunits
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Summary The marine red algaErythrocladia subintegra synthesizes cellulose microfibrils as determined by CBH I-gold labelling, X-ray and electron diffraction analyses. The cellulose microfibrils are quite thin, ribbon-like structures, 1–1.5 nm in thickness (constant), and 10–33 nm in width (variable). Several laterally associated minicrystal components contribute to the variation in microfibrillar width. Electron diffraction analysis suggested a uniplanar orientation of the microfibrils with their (101) lattice planes parallel to the plasma membrane surface of the cell. The linear particle arrays bound in the plasma membrane and associated with microfibril impressions recently demonstrated inErythrocladia have been shown in this study to be the cellulose-synthesizing terminal complexes (TCs). The TCs appear to be organized by a repetition of transverse rows consisting of four TC subunits, rather than by four rows of longitudinallyarranged TC subunits. The number of transverse rows varied between 8–26, corresponding with variation in the length of the TCs and the width of the microfibrils. The spacings between the neighboring transverse rows are almost constant being 10.5–11.5 nm. Based on the knowledge thatAcetobacter, Vaucheria, andErythrocladia synthesize similar thin, ribbon-like cellulose microfibrils, the structural characteristics common to the organization of distinctive TCs occurring in these three organisms has been discussed, so that the mode of cellulose microfibril assembly patterns may be deciphered.
    Type of Medium: Electronic Resource
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