Biosynthesis of cellulose in red algae

Abstract

There is now ample evidence that cellulose biosynthesis occurs at the plasma membrane-bound cellulose synthase [1,2]. Freeze-fracture studies of the supramolecular organization of the plasma membrane support the view that the rosettes (a six-subunit complex) in higher plants and both rosettes and the linear terminal complexes (TCs) in algae are the structures that synthesize cellulose and secrete cellulose microfibrils [1, 3]. One typical feature of the linear terminal complexes in red algae is the periodic arrangement of the particle rows transverse to the longitudinal axis of the TCs “Figures 1, 2”. In bangiophyte red algae cellulose microfibrils are thin, ribbon-shaped structures, 1–1.5 nm thick and 5–70 nm wide “Figure 3”. Terminal complexes appear to be made in the endoplasmic reticulum and are transferred as large globular particles (globules, precursor units) to Golgi cisternae, where the cellulose synthases are activated and may be transported to the plasma membrane“Figure 4” [1, 3]. The globules are composed of a central hole (the core) with small subunits forming a peripheral ridge and seem to represent zymogenic precursors “Figure 5”. In the plasma membrane the globules (consisting of more subunits) aggregate, swell and unfold, and become closely arranged contributing to TC assembly “Figure 5” [3]. 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 (double row-formation centers) [3]. In the red algae with linear TCs, deposition of microfibrils follows a precise pattern directed by the movement and the orientation of the TCs (membrane flow). A principal underlying theme is that the architecture of cellulose microfibrils (size, shape, crystallinity, and intramicrofibrillar associations) is directly related to the geometry of TCs.