The insoluble granular material of the acrosome vesicle of sea urchin sperm has been isolated and shown to be a single 30,500 dalton protein for which the name "bindin" is proposed. The data presented are consistent with the hypothesis that bindin is the adhesive responsible for the attachment of sperm to the vitelline layer of the egg. Experimental results suggest that bindin may act by binding to carbohydrate receptors of vitelline layer glycoproteins. The speculation is made that sperm bindins may be the general mechanism by which animal sperm attach to eggs.

https://www.pnas.org/content/pnas/74/6/2456.full.pdf

Isolation of bindin: the protein responsible for adhesion of sperm to sea urchin eggs

The phylogenetic relationships within the kingdom Animalia (Metazoa) have long been questioned. Focusing on the lowest eukaryotic multicellular organisms, the metazoan phylum Porifera (sponges), it remained unsolved if they evolved multicellularity independently from a separate protist lineage (polyphyly of animals) of derived from the same protist group as the other animal phyla (monophyly). After having analyzed genes typical for multicellularity (adhesion molecules/receptors and a nuclear receptor), we present evidence that Porifera should be placed in the kingdom Animalia. We therefore suggest a monophyletic origin for all animals.

Molecular phylogeny of metazoa (animals): Monophyletic origin

The siliceous skeleton of demosponges is constructed of spicules. We have studied the formation of spicules in primmorphs from Suberites domuncula. Scanning electron microscopy and transmission electron-microscopical (TEM) analyses have revealed, in the center of the spicules, an axial canal that is 0.3–1.6 μm wide and filled with an axial filament. This filament is composed of the enzyme silicatein, which synthesizes the spicules. TEM analysis has shown that spicule formation starts intracellularly and ends extracellularly in the mesohyl. At the initial stage, the axial canal is composed only of silicatein, whereas membranous structures and fibrils (10–15 nm in width) can later also be identified, suggesting that intracellular components protrude into the axial canal. Antibodies against silicatein have been applied for Western blotting; intracellularly, silicatein is processed to the mature form (24 kDa), whereas the pro-enzyme with the propeptide (33 kDa) is detected extracellularly. Silicatein undergoes phosphorylation at five sites. Immunohistological analysis has shown that silicatein exists in the axial canal (axial filament) and on the surface of the spicules, suggesting that they grow by apposition. Finally, we have demonstrated that the enzymic reaction of silicatein is inhibited by anti-silicatein antibodies. These data provide, for the first time, a comprehensive outline of spicule formation.

Formation of siliceous spicules in the marine demosponge Suberites domuncula.

The origin of Metazoa remained — until recently — the most enigmatic of all phylogenetic problems. Sponges [Porifera] as ‘living fossils’, positioned at the base of multicellular animals, have been used to answer basic questions in metazoan evolution by molecular biological techniques. During the last few years, cDNAs/genes coding for informative proteins have been isolated and characterized from sponges, especially from the marine demosponges Suberites domuncula and Geodia cydonium. The analyses of their deduced amino acid sequences allowed a molecular biological resolution of the monophyly of Metazoa. Molecules of the extracellular matrix/basal lamina, with the integrin receptor, fibronectin and galectin as prominent examples, cell-surface receptors (tyrosine kinase receptors), elements of nerve system/sensory cells (metabotropic glutamate receptor), homologs/modules of an immune system [immunoglobulin-like molecules, SRCR- and SCR-repeats, cytokines, (2–5)A synthetase], as well as morphogens (myotrophin) classify the Porifera as true Metazoa. As ‘living fossils’, provided with simple, primordial molecules allowing cell–cell and cell–matrix adhesion, as well as processes of signal transduction as known in a more complex manner from higher Metazoa, sponges also show peculiarities. Tissues of sponges are rich in telomerase activity, suggesting a high plasticity in the determination of cell lineages. It is concluded that molecular biological studies with sponges as models will not only help to understand the evolution to the Metazoa but also the complex, hierarchical regulatory network of cells in higher Metazoa [reviewed in Progress in Molecular Subcellular Biology, vols. 19, 21 (1998) Springer Verlag]. The hypothetical ancestral animal, the Urmetazoa, from which the metazoan lineages diverged (more than 600 MYA), may have had the following characteristics: cell adhesion molecules with intracellular signal transduction pathways, morphogens/growth factors forming gradients, a functional immune system, and a primordial nerve cell/receptor system.

Review: How was metazoan threshold crossed? The hypothetical Urmetazoa.

The integrin family of cell surface receptors is strongly conserved in higher animals, but the evolutionary history of integrins is obscure. We have identified and sequenced cDNAs encoding integrin β subunits from a coral (phylum Cnidaria) and a sponge (Porifera), indicating that these proteins existed in the earliest stages of metazoan evolution. The coral βCn1 and, especially, the sponge βPo1 sequences are the most divergent of the “β1-class” integrins and share a number of features not found in any other vertebrate or invertebrate integrins. Perhaps the greatest difference from other β subunits is found in the third and fourth repeats of the cysteine-rich stalk, where the generally conserved spacings between cysteines are highly variable, but not similar, in βCn1 and βPo1. Alternatively spliced cDNAs, containing a stop codon about midway through the full-length translated sequence, were isolated from the sponge library. These cDNAs appear to define a boundary between functional domains, as they would encode a protein that includes the globular ligand-binding head but would be missing the stalk, transmembrane, and cytoplasmic domains. These and other sequence comparisons with vertebrate integrins are discussed with respect to models of integrin structure and function.

https://www.pnas.org/content/pnas/94/17/9182.full.pdf

Molecular evolution of integrins: Genes encoding integrin β subunits from a coral and a sponge

SUMMARY An aggregation receptor from the siliceous sponge Geodia cydonium has been isolated and purified in an almost pure form. It sediments at about 2-6 s, has a buoyant density of 151 g/ml in CsCl and elutes from Sephadex G-50 at a V,jV0 value of i - 3ii . Chemical analysis revealed that the receptor consists of 81 % neutral carbohydrate and 7-5% protein. The activity of the receptor is rapidly destroyed by Na-periodate. The receptor is released from the cell surface after removal of Ca 2+ from the medium or after incubation of the cells with trypsin. The depleted cells can be charged again with isolated receptor molecules. The binding of the receptor molecules on the cell surface is prevented in the presence of trypsin. For optimal binding, physiological salt concentrations with respect to NaCl (540 mM NaCl) and Ca !+ ions are necessary. The receptor whose isolation is described in this report, is involved in secondary aggregation processes, which are initiated by a soluble aggregation factor. The primary aggregation of the cells is not influenced by the receptor. Time-course studies with receptor-depleted cells revealed that new aggregation receptor molecules are formed during the aggregation process. By competition experiments it could be shown that high concentrations of soluble aggregation receptor molecules inhibit secondary aggregation. The soluble receptor molecules can compete with surface-bound receptor molecules only if these are not linked with the aggregation factor.

Species-specific aggregation factor in sponges vi. aggregation receptor from the cell surface

Aggregation of sponge cells. A novel mechanism of controlled intercellular adhesion, basing on the interrelation between glycosyltransferases and glycosidases.

Role of the aggregation factor in the regulation of phosphoinositide metabolism in sponges. Possible consequences on calcium efflux and on mitogenesis.

Collagen, isolated from the sponge Geodia cydonium in the absence of denaturing agents, had the typical amino acid composition and was associated with the carbohydrates galactose and glucose. The resulting individual fibrils with a diameter of 23 nm, displayed a 19.5 nm periodicity with one intraperiod band. A collagen assembly factor (CAF) was identified in and partially purified from the extracellular space. The CAF reacted with antibodies against intact Geodia cells but not with antibodies against Geodia lectin and Geodia aggregation factor. In the presence of the CAF, the collagen fibrils reconstituted collagen bundles in an ordered sequence of events, which were followed by electron-microscopical and biochemical methods. Bundle formation was not dependent on the presence of the homologous lectin, glycoconjugates or aggregation factor. Homologous cells (Geodia archaeocytes) were determined to attach only to those Geodia collagen substrates that contained CAF. The attachment of these cells did not require fibronectin or Geodia lectin. Homologous glycoconjugates or NaOH-treated collagen inhibited cell attachment. Collagen from the sponge Chondrosia reniformis, even in the presence of Geodia CAF, was no appropriate substrate for Geodia cell attachment. Whether collagen is a component of cell-matrix interactions in sponge systems also in vivo is discussed.

Attachment of sponge cells to collagen substrata: effect of a collagen assembly factor.

Aggregation of sponge cells. Function of a lectin in its homologous biological system.

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