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Hydroxyproline-rich glycoproteins form an important, but little understood, structural component of most cell walls. Their occurrence, chemistry, synthesis, secretion, cross-linking and functions in higher plant cell walls will be briefly reviewed. Similar molecules also occur in other groups of plants; in particular, in the algae. In many of these they form highly ordered cell surface arrays, and we have studied these by high-resolution electron microscopy and computer image reconstruction. Some resulting three-dimensional models of these are presented. One particular glycoprotein, the major structural component of the cell wall of Chlamydonwnas reinhardii, has been investigated in some detail. The chemistry and structure of this glycoprotein, which we have called volvin, has. been studied and a family of monoclonal antibodies has been raised against it. Some of these antibodies appear to be specific to oligosaccharide side-chains and allow the localization of these substituents and their sites of synthesis. Immunofluorescence studies have shown that the expression of some of these antigenic determinants is developmentally regulated or cell-cycle-dependent. Immunogold labelling of thin sections has enabled the sites of synthesis and the method of secretion to be determined. These results will be discussed in the context of other cell wall glycoproteins, their relation to other glycoproteins, such as the mating agglutinin, and to their possible functions.

Members of the Chlamydomonaceae, mostly single-celled green algae, have been shown to contain a crystalline glycoprotein cell wall component. Most of the species examined fall into a class of algae whose walls have an identical crystalline unit cell. Chlorogonium elongatum has been chosen as a representative of this class in order to investigate in more detail its cell wall structure. The alga has a spindleshaped cell wall which retains its asymmetric shape on isolation. Sections from walls fixed in the presence of tannic acid clearly reveal a regular subunit monolayer, about 20 nm thick, within the wall. Sodium dodecylsulphate (SDS) polyacrylamide gel electrophoresis shows the presence of at least 2 major glycoprotein species in the wall. Negatively stained purified cell walls demonstrate the crystalline nature of the cell wall. Optical diffraction of bright-field images and direct electron diffraction both give clear diffraction patterns whose spacings extend out to 3 nm and fall on a reciprocal lattice whose vectors describe a 2-dimensional unit cell within the wall 2 · 15 nm × 7 · 0 nm and an included angle of 80°. Lattice defects within the cell wall are revealed by both negative staining and surface replication. Through-focal series were used to choose images with the optimal degree of underfocus for image processing. Linear integration and optical filtering of such images gave essentially the same result. A similar image was also obtained by computing the autocorrelation function of the amplitudes in the electron-diffraction pattern and the optical-diffraction pattern of the in-focus image. On the basis of these data a 2-dimensional model of the crystalline cell wall layer is presented.

The green alga Chlorogonium elongatum, a member of the Volvocales, possesses a crystalline cell wall composed of hydroxyproline-rich glycoprotein similar to the primary cell wall glycoproteins of higher plants. Electron microscopy and computer image processing have been used to determine the crystal structure of the Chlorogonium cell wall in three dimensions to a resolution of 2·Onm. The structure is composed of heterologous dimers. Each subunit of the dimer comprises a long, thin spacer domain and a large globular domain, which is the site of the intra- and inter-dimer interactions. There are also sites of intersubunit interactions at the opposite ends of the rod domains. We suggest that the rods are composed predominantly of glycosylated polyproline helix, as has been suggested for higher plant cell wall glycoproteins and has been shown for the cell wall glycoprotein of Chlamydomonas reinhardtii, which is closely related to Chlorogonium.

Lobomonas piriformis is a member of an order of green algae (Volvocales) that have crystalline glycoprotein cell walls. As part of a program of investigation of these glycoproteins and their architecture we have studied the cell wall of Lobomonas by a variety of chemical, electron- microscopical and image-analysis techniques. Lobomonas and Vitreochlamys incisa show a very similar structure in their cell walls and represent 1 of the 4 classes into which all the structures of the walls of these algae that we have so far examined fall. The 2 classes that we have previously studied in detail, represented by Chlamydomonas reinhardii and Chlorogonium elongatum, have a crystalline component of the wall that is a more or less smooth continuous surface overlying an amorphous inner wall layer. Although Lobomonas also has this 2-layer structure, the crystalline layer consists of distinct plates, each of which is built around a single, very coherent crystal lattice. The polar nature of the architecture of the cell wall is shown by sectioning and by examination of the cell-wall surface by metal-shadowing of carbon replicas, both of intact cells and of isolated cell-wall plates. There are great similarities in chemical composition between the glycoproteins of the cell wall of C. reinhardii and those of Lobomonas. Both have a large content of hydroxyproline in their amino acid composition and a sugar/hydroxyproline ratio of about 6·0, and both contain sugar sulphates. Lobomonas however has a large glucose content, whereas Chlamydomonas has almost none. Electron micrographs of walls stained with methylamine tungstate and shadowed specimens show that the Lobomonas crystal structure is entirely different from that of C. reinhardii, and that there is a distinctly different structure in the centre of the plates from that at their edges, although the transition between the 2 areas occurs with no distortion of the crystal lattice. Computer image analysis has been used to calculate reconstructed images of the 2 areas, and by using minimal-dose techniques has yielded 2-dimensional maps of the negatively stained structure at a resolution of 1·8 nm. The 2-sided plane group of both areas of the crystal is P2, and the centre area contains 2 distinct structural units, both centred on dyad axes, together with other more complex features. In the edge structure, one of the structural units appears unchanged, but the other unit has a considerably different appearance. The most likely interpretation of this is as a conformational or positional change in one of the subunits. However, because the underlying lattice is so accurately maintained across this transition, it seems probable that the basic structural arrangement that defines the lattice is common to the 2 areas. Some of the computational and mathematical techniques used in the image analysis have not been previously published and are described in detail and compared with published techniques in an Appendix.