Species-specific aggregation factor in sponges. Sialyltransferase associated with aggregation factor.

The sialyltransferase (= glycoprotein-sialic acid transferase) was studied in the sponge Geodia cydonium, a mesozoan organism. The experiments were performed both in intact cellular and in isolated enzyme systems. It is shown, that desialylated cells show a lower aggregation potency than the controls. During aggregation enzymic sialylation of desialylated sponge cells occurs in the presence of an aggregation factor, which is associated with a high molecular weight particle. The sialylation process is temperature-dependent and can be inhibited by N-ethylmaleimide. Sialylation occurs predominantly at a distinct cell surface component, the aggregation receptor. The sialyltransferase was isolated and purified by the following steps: Sepharose 4B, CM-cellulose, Nonidet treatment, and Sephadex G-100. By this procedure the enzyme was purified 680-fold with a 31% yield. The sialyltransferase is originally associated with the high molecular weight particle also carrying the aggregation factor. In the last step the aggregation factor was separated from the sialyltransferase. The enzyme catalyzes the transfer of sialic acid from CMP-sialic acid to the desialylated aggregation receptor. The molecular weight of the sialyltransferase has been determined to be 52,000. Kinetic studies revealed no lag phase and a dependence on enzyme concentration. The purified transferase has a pH optimum of 7.75 and requires 200 mM NaCl for activity. No requirement for Mg2+ or Ca2+ could be observed. The reaction is inhibited by 10 micronM N-ethylmaleimide.     

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

For the first time, the biological role of a lectin in the process of reaggregation of single cells from the same species (marine sponge: Geodia cydonium Jam.) is described. The galactose-specific lectin does not promote aggregation, but prevents the antiaggregation receptor from disaggregating cell clumps. Competition experiments showed that the lectin inactivates the antiaggregation receptor by binding to it, most likely via its terminal galactose residues. The lectin converts reversibly aggregation-deficient cells (carrying functional cell membrane-bound antiaggregation receptor molecules) to aggregation-susceptible cells.     

Species-specific aggregation factor in sponges. XIII. Entire and core structure of the large circular proteid particle from Geodia cydonium

High-molecular weight particles have been isolated from the sponge Geodica cydonium. In the "native" from these particles consist of a spherical center and have 25-30 filaments attached to it. The core structure of the particles is assembled of a central circle and 25 radially-arranged filaments. The core structure is obtained from the entire structure by incubation in a medium, containing a non-ionic detergent and EDTA. The molecular weight of the enitre structure was in the range of 1.4 X 10(9) daltons or more and of the core structure 6.1 x 10(8) daltons. Two functional proteins are released from the "native" particles: the aggregation factor and the sialytransferase.     

Aggregation of sponge cells. XX. Self-aggregation of the circular proteid particle.

In the extracellular space of the tissue of the sponge Geodia cydonium, circular proteid particles are found which carry as subunits the aggregation factor and a series of glycosyltransferases. Using the technique of velocity sucrose gradient centrifugation, the sedimentation coefficient (S020,w) of the particle-monosomes was determined to be 90. By means of the Svedberg equation a molecular weight of 1.3 . 10(8) daltons could be estimated. The monosomes aggregate in the presence of Ca2+ to higher complexes via disomes, trisomes, and pentasomes. The complexes can be redissociated by dodecyl sulfate but not by EDTA. During the Ca2+-mediated self-aggregation, the particles lose their biological activity with respect to their aggregation promoting function.     

Cell junctions in the epithelium of Bowman's capsule

Summary The intercellular connections between the epithelial cells of Bowman's capsule were investigated. It could be demonstrated that typical zonulae occludentes (tight junctions) are present in the species (rat, hamster, and Tupaia) studied. Freeze-fracturing shows a network of anastomizing strands; some species variations are described. In the rat two strands are common. In the golden hamster mostly two to four and occasionally five strands occur. In Tupaia regularly three tight junction strands are found and also gap junctions associated with the zonulae occludentes. In thin sections the goniometric analysis confirms the freeze-fracturing results and reveals attachment zones of macular shape, which are classified as intermediate junctions and desmosomes. The functional role of these cell junctions observed in the epithelium of Bowman's capsule is discussed.