Domain conservation in several volvocalean cell wall proteins

The cell-wall glycoproteins of the green alga Scenedesmus obliquus. The predominant cell-wall polypeptide of Scenedesmus obliquus is related to the cell-wall glycoprotein gp3 of Chlamydomonas reinhardtii

The green alga Scenedesmus obliquus contains a multilayered cell wall, ultrastructurally similar to that of Chlamydomonas reinhardtii, although its proportion of hydroxyproline is considerably lower. Therefore, we have investigated the polypeptide composition of the insoluble and the chaotrope-soluble wall fractions of S. obliquus. The polypeptide pattern of the chaotrope-soluble wall fraction was strongly modified by chemical deglycosylation with anhydrous hydrogen fluoride (HF) in pyridine indicating that most of these polypeptides are glycosylated. Polypeptide constituents of the chaotrope-soluble cell-wall fraction with apparent molecular masses of 240, 270, 265, and 135 kDa cross-reacted with a polyclonal antibody raised against the 100 kDa deglycosylation product of the C. reinhardtii cell-wall glycoprotein GP3B. Chemical deglycosylation of the chaotrope-soluble wall fraction resulted in a 135 kDa major polypeptide and a 106 kDa minor component reacting with the same antibody. This antibody recognized specific peptide epitopes of GP3B. When the insoluble wall fraction of S. obliquus was treated with anhydrous HF/pyridine, three polypeptides with apparent molecular masses of 144, 135, and 65 kDa were solubilized, which also occured in the deglycosylated chaotrope-soluble wall fraction. These findings indicate that theses glycoproteins are cross-linked to the insoluble wall fraction via HF-sensitive bonds.

A COMPARATIVE ANALYSIS OF THE VOLVOCACEAE (CHLOROPHYTA)(1)

This review covers essentially all aspects of the organisms in the green algal family Volvocaceae and suggests the genetic history of the various steps in their evolution from their unicellular ancestors.

Diverse evolutionary paths to cell adhesion

The morphological diversity of animals, fungi, plants, and other multicellular organisms stems from the fact that each lineage acquired multicellularity independently. A prerequisite for each origin of multicellularity was the evolution of mechanisms for stable cell-cell adhesion or attachment. Recent advances in comparative genomics and phylogenetics provide critical insights into the evolutionary foundations of cell adhesion. Reconstructing the evolution of cell junction proteins in animals and their unicellular relatives exemplifies the roles of co-option and innovation. Comparative studies of volvocine algae reveal specific molecular changes that accompanied the evolution of multicellularity in Volvox. Comparisons between animals and Dictyostelium show how commonalities and differences in the biology of unicellular ancestors influenced the evolution of adhesive mechanisms. Understanding the unicellular ancestry of cell adhesion helps illuminate the basic cell biology of multicellular development in modern organisms.

CELL WALL CARBOHYDRATE EPITOPES IN THE GREEN ALGA OEDOGONIUM BHARUCHAE F. MINOR (OEDOGONIALES, CHLOROPHYTA)(1)

Cell wall changes in vegetative and suffultory cells (SCs) and in oogonial structures from Oedogonium bharuchae N. D. Kamat f. minor Vélez were characterized using monoclonal antibodies against several carbohydrate epitopes. Vegetative cells and SCs develop only a primary cell wall (PCW), whereas mature oogonial cells secrete a second wall, the oogonium cell wall (OCW). Based on histochemical and immunolabeling results, (1→4)-β-glucans in the form of crystalline cellulose together with a variable degree of Me-esterified homogalacturonans (HGs) and hydroxyproline-rich glycoprotein (HRGP) epitopes were detected in the PCW. The OCW showed arabinosides of the extensin type and low levels of arabinogalactan-protein (AGP) glycans but lacked cellulose, at least in its crystalline form. Surprisingly, strong colabeling in the cytoplasm of mature oogonia cells with three different antibodies (LM-5, LM-6, and CCRC-M2) was found, suggesting the presence of rhamnogalacturonan I (RG-I)-like structures. Our results are discussed relating the possible functions of these cell wall epitopes with polysaccharides and O-glycoproteins during oogonium differentiation. This study represents the first attempt to characterize these two types of cell walls in O. bharuchae, comparing their similarities and differences with those from other green algae and land plants. This work represents a contribution to the understanding of how cell walls have evolved from simple few-celled to complex multicelled organisms.

Structural analysis of linear hydroxyproline-bound O-glycans of Chlamydomonas reinhardtii--conservation of the inner core in Chlamydomonas and land plants

Linear hydroxyproline-bound O-glycans of the unicellular green alga Chlamydomonas reinhardtii were isolated from outer cell wall glycoproteins and their structure elucidated by chemical and spectroscopic methods. They consist exclusively of arabinose and galactose, the latter in the furanose form, unusual for plants. The first two arabinoses linked to hydroxyproline have the same anomeric configuration and linkage to each other as do the arabinosides isolated from land plants, suggesting that these two steps of hydroxyproline-O-glycosylation have been conserved during evolution. In a concomitant analysis of Hyp-bound O-tri- and tetraarabinans isolated from Arabidopsis thaliana, we were able to unambiguously identify the natural trans-4-l-Hyp-bound O-glycans and the artificially produced d-allo forms and to identify diagnostic signals in NMR spectra of these compounds for the first time.

The chaotrope-soluble glycoprotein GP2 is a precursor of the insoluble glycoprotein framework of the Chlamydomonas cell wall

The cell wall of the unicellular green alga Chlamydomonas reinhardtii consists of an insoluble, hydroxyproline-rich glycoprotein framework and several chaotrope-soluble, hydroxyproline-containing glycoproteins. Up to now, there have been no data concerning the amino acid sequences of the hydroxyproline-containing polypeptides of the insoluble wall fraction. Matrix-assisted laser desorption ionization time-of-flight analyses of peptides released from the insoluble cell wall fraction by trypsin treatment revealed the presence of 14 peptide fragments that could be attributed to non-glycosylated domains of the chaotrope-soluble cell wall glycoprotein GP2. However, these peptides cover only 15% of the GP2 polypeptide backbone. Considerably more information concerning the presence of GP2 in the insoluble cell wall fraction was obtained by an immunochemical approach. For this purpose, 407 overlapping pentadecapeptides covering the whole known amino acid sequence of GP2 were chemically synthesized and probed with a polyclonal antibody raised against the deglycosylated, insoluble cell wall fraction. This particular antibody reacted with 297 of the 407 GP2-derived peptides. The peptides that were recognized by this antibody are distributed over the whole known GP2 sequence. The epitopes recognized by polyclonal antibodies raised against the 64- and 45-kDa constituents purified from the deglycosylation products of the insoluble cell wall fraction are also distributed over the whole GP2 backbone, although the corresponding antigens are considerably smaller than GP2. The significance of the latter results for the structure of the insoluble cell wall fraction is discussed.

Between-species analysis of short-repeat modules in cell wall and sex-related hydroxyproline-rich glycoproteins of Chlamydomonas

Protein diversification is commonly driven by single amino acid changes at random positions followed by selection, but, in some cases, the structure of the gene itself favors the occurrence of particular kinds of mutations. Genes encoding hydroxyproline-rich glycoproteins (HRGPs) in green organisms, key protein constituents of the cell wall, carry short-repeat modules that are posited to specify proline hydroxylation and/or glycosylation events. We show here, in a comparison of two closely related Chlamydomonas species-Chlamydomonas reinhardtii (CC-621) and Chlamydomonas incerta (CC-1870/3871)-that these modules are prone to misalignment and hence to both insertion/deletion and endoduplication events, and that the dynamics of the rearrangements are constrained by purifying selection on the repeat patterns themselves, considered either as helical or as longitudinal face modules. We suggest that such dynamics may contribute to evolutionary diversification in cell wall architecture and physiology. Two of the HRGP genes analyzed (SAG1 and SAD1) encode the mating-type plus and minus sexual agglutinins, displayed only by gametes, and we document that these have undergone far more extensive divergence than two HRGP genes (GP1 and VSP3) that encode cell wall components-an example of the rapid evolution that characterizes sex-related proteins in numerous lineages. Strikingly, the central regions of the agglutinins of both mating types have diverged completely, by selective endoduplication of repeated motifs, since the two species last shared a common ancestor, suggesting that these events may have participated in the speciation process.

Mating-induced shedding of cell walls, removal of walls from vegetative cells, and osmotic stress induce presumed cell wall genes in Chlamydomonas

The first step in sexual differentiation of the unicellular green alga Chlamydomonas reinhardtii is the formation of gametes. Three genes, GAS28, GAS30, and GAS31, encoding Hyp-rich glycoproteins that presumably are cell wall constituents, are expressed in the late phase of gametogenesis. These genes, in addition, are activated by zygote formation and cell wall removal and by the application of osmotic stress. The induction by zygote formation could be traced to cell wall shedding prior to gamete fusion since it was seen in mutants defective in cell fusion. However, it was absent in mutants defective in the initial steps of mating, i.e. in flagellar agglutination and in accumulation of adenosine 3',5'-cyclic monophosphate in response to this agglutination. Induction of the three GAS genes was also observed when cultures were exposed to hypoosmotic or hyperosmotic stress. To address the question whether the induction seen upon cell wall removal from both gametes and vegetative cells was elicited by osmotic stress, cell wall removal was performed under isosmotic conditions. Also under such conditions an activation of the genes was observed, suggesting that the signaling pathway(s) is (are) activated by wall removal itself.

Plus and minus sexual agglutinins from Chlamydomonas reinhardtii

Gametes of the unicellular green alga Chlamydomonas reinhardtii undergo sexual adhesion via enormous chimeric Hyp-rich glycoproteins (HRGPs), the plus and minus sexual agglutinins, that are displayed on their flagellar membrane surfaces. We have previously purified the agglutinins and analyzed their structural organization using electron microscopy. We report here the cloning and sequencing of the Sag1 and Sad1 genes that encode the two agglutinins and relate their derived amino acid sequences and predicted secondary structure to the morphology of the purified proteins. Both agglutinin proteins are organized into three distinct domains: a head, a shaft in a polyproline II configuration, and an N-terminal domain. The plus and minus heads are related in overall organization but poorly conserved in sequence except for two regions of predicted hydrophobic alpha-helix. The shafts contain numerous repeats of the PPSPX motif previously identified in Gp1, a cell wall HRGP. We propose that the head domains engage in autolectin associations with the distal termini of their own shafts and suggest ways that adhesion may involve head-head interactions, exolectin interactions between the heads and shafts of opposite type, and antiparallel shaft-shaft interactions mediated by carbohydrates displayed in polyproline II configurations.

Cloning and characterization of novel extensin-like cDNAs that are expressed during late somatic cell phase in the green alga Volvox carteri

Asexual individuals of the green alga Volvox carteri consist of two cell types, somatic and reproductive cells. The somatic cells are terminally differentiated post-mitotic cells which undergo gradual senescence leading to cell death in every generation. To understand the gene expression programs associated with senescence of somatic cells, we cloned two cDNAs, LSG1 and LSG2, that are preferentially expressed during this late developmental stage. These two cDNAs were deduced to encode Pro-rich motifs characteristic of extensin proteins that are components of the extracellular matrix. LSG1 also resembled genes encoding plant pathogenesis-related protein 1 (PR-1), while LSG2 showed similarities with genes encoding matrix metalloproteinases, including a gamete lytic enzyme of Chlamydomonas. We also found that S9, one of the late somatic cDNAs previously cloned by Tam and Kirk (Dev. Biol. 145 (1991) 51), was deduced to encode a protein with a composition similar to LSG2. The expression of PR-1 and a matrix-metalloproteinase-encoding gene has been shown to be induced during senescence in higher plants. These results indicate that some of the late somatic genes in V. carteri are related to the senescence-associated genes in higher plants.

Sulfur economy and cell wall biosynthesis during sulfur limitation of Chlamydomonas reinhardtii

We have identified two novel periplasmic/cell wall polypeptides that specifically accumulate during sulfur limitation of Chlamydomonas reinhardtii. These polypeptides, present at high levels in the extracellular polypeptide fraction from a sulfur-deprived, cell wall-minus C. reinhardtii strain, have apparent molecular masses of 76 and 88 kD and are designated Ecp76 and Ecp88. N-terminal sequences of these polypeptides facilitated the isolation of full-length Ecp76 and Ecp88 cDNAs. Ecp76 and Ecp88 polypeptides are deduced to be 583 and 595 amino acids, respectively. Their amino acid sequences are similar to each other, with features characteristic of cell wall-localized hydroxyproline-rich glycoproteins; the N terminus of each polypeptide contains a predicted signal sequence, whereas the C terminus is rich in proline, alanine, and serine. Ecp76 and Ecp88 have either no (Ecp88) or one (Ecp76) sulfur-containing amino acid and transcripts encoding these polypeptides are not detected in cultures maintained on complete medium, but accumulate when cells are deprived of sulfur. This accumulation is temporally delayed relative to the accumulation of sulfur stress-induced arylsulfatase and ATP sulfurylase transcripts. The addition of sulfate back to sulfur-starved cultures caused a rapid decline in Ecp76 and Ecp88 mRNAs (half lives < 10 min). Furthermore, the C. reinhardtii sac1 mutant, which lacks a regulatory protein critical for acclimation to sulfur limitation, does not accumulate Ecp76 or Ecp88 transcripts. These results suggest that the Ecp76 and Ecp88 genes are under SacI control, and that restructuring of the C. reinhardtii cell wall during sulfur limitation may be important for redistribution of internal and efficient utilization of environmental sulfur-containing molecules.

The developmentally regulated ECM glycoprotein ISG plays an essential role in organizing the ECM and orienting the cells of Volvox

Volvox is one of the simplest multicellular organisms with only two cell types, yet it has a surprisingly complex extracellular matrix (ECM) containing many region-specific morphological components, making Volvox suitable as a model system for ECM investigations. ECM deposition begins shortly after inversion, which is the process by which the embryo turns itself right-side-out at the end of embryogenesis. It was previously shown that the gene encoding an ECM glycoprotein called ISG is transcribed very transiently during inversion. Here we show that the developmentally controlled ISG accumulates at the bases of the flagella right after inversion, before any morphologically recognizable ECM structures have yet developed. Later, ISG is abundant in the ‘flagellar hillocks’ that encircle the basal ends of all flagella, and in the adjacent ‘boundary zone’ that delimits the spheroid. Transgenic Volvox were generated which express a truncated form of ISG. These transgenics exhibit a severely disorganized ECM within which the cells are embedded in a highly chaotic manner that precludes motility. A synthetic version of the C-terminal decapeptide of ISG has a similar disorganizing effect, but only when it is applied during or shortly after inversion. We postulate that ISG plays a critical role in morphogenesis and acts as a key organizer of ECM architecture; at the very beginning of ECM formation ISG establishes an essential initial framework that both holds the somatic cells in an adaptive orientation and acts as the scaffold upon which the rest of the ECM can be properly assembled, assuring that somatic cells of post-inversion spheroids are held in orientations and locations that makes adaptive swimming behavior possible.

A ZYGOTE-SPECIFIC PROTEIN WITH HYDROXYPROLINE-RICH GLYCOPROTEIN DOMAINS AND LECTIN-LIKE DOMAINS INVOLVED IN THE ASSEMBLY OF THE CELL WALL OF CHLAMYDOMONAS REINHARDTII (CHLOROPHYTA)

The cell wall of Chlamydomonas reinhardtii zygotes, which forms rapidly after the fusion of wall-free gametes, provides a tractable system for studying the properties and assembly of hydroxyproline-rich glycoproteins, the major proteinaceous components of green algal and plant cell walls. We report the cloning of the zsp2 gene and the analysis of its ZSP-2 product, a 58.9 kDa polypeptide that is synthesized exclusively by zygotes. The protein contains two (SP)_x repeats, establishing it as a member of the cell wall hydroxyproline-rich glycoproteins family. It also contains a 4-fold iteration of an amino acid sequence centered around cysteine residues, a configuration found in both plant and animal lectins. Furthermore, we report four observations on pellicle composition and production. First, cell-free preparations of the pellicle matrix are rich in hydroxyproline, arabinose, and galactose and contain bundles of very long fibrils. Second, glutathione blocks pellicle formation and results in the accumulation of long fibrils in the growth medium. Third, antibody to ZSP-2 also blocks pellicle formation. Fourth, ZSP-2 immunolocalizes to the boundary between the outer layers of the wall proper and the pellicle matrix. These observations are consistent with the possibility that the Cys-rich (glutathione-sensitive) lectin-like domains of ZSP-2 may bind to sugar residues on the long fibrils and anchor them to the cell wall, thereby initiating and maintaining pellicle formation.

PLANT CELL WALL PROTEINS

The nature of cell wall proteins is as varied as the many functions of plant cell walls. With the exception of glycine-rich proteins, all are glycosylated and contain hydroxyproline (Hyp). Again excepting glycine-rich proteins, they also contain highly repetitive sequences that can be shared between them. The majority of cell wall proteins are cross-linked into the wall and probably have structural functions, although they may also participate in morphogenesis. On the other hand, arabinogalactan proteins are readily soluble and possibly play a major role in cell-cell interactions during development. The interactions of these proteins between themselves and with other wall components is still unknown, as is how wall components are assembled. The possible functions of cell wall proteins are suggested based on repetitive sequence, localization in the plant body, and the general morphogenetic pattern in plants.

Biochemistry of the extracellular matrix of Volvox

The volvocine algae range in complexity from unicellular Chlamydomonas to multicellular organisms in the genus Volvox. The transition from unicellularity to multicellularity in the Volvocales is a recent event in evolution. Thus, these organisms provide a unique opportunity for exploring the development of a complex extracellular matrix (ECM) from the cell wall of a unicellular ancestor. The ECM of Volvox is divided into four main zones: The flagellar, boundary, cellular, and deep zones. Each zone is defined by ultrastructure and by characteristic ECM glycoproteins. Volvox ECM is modified under developmental control or in response to external stimuli, like the sex-inducing pheromone or stress factors. The structures of more than 10 ECM glycoproteins from a single species of Volvox are now known in molecular detail and are compared to other algal and plant cell wall/ECM glycoproteins. Although usually classified as hydroxyproline-rich glycoproteins, the striking feature of all algal ECM glycoproteins is a modular composition. Rod-shaped hydroxyproline-rich modules are combined with hydroxyproline-free domains that meet the multiple functional requirements of a complex ECM. The algal ECM provides another example of the combinatorial advantage of shuffling modules that is so evident in the evolution of the metazoan ECMs.