Spectroscopical studies on the structural organization of the lectin discoidin I: analysis of sugar- and calcium-binding activities

Characterization and purification of the discoidin domain-containing protein retinoschisin and its interaction with galactose

RS1, also known as retinoschisin, is an extracellular discoidin domain-containing protein that has been implicated in maintaining the cellular organization and synaptic structure of the vertebrate retina. Mutations in the gene encoding RS1 are responsible for X-linked retinoschisis, a retinal degenerative disease characterized by the splitting of the retinal cell layers and visual impairment. To better understand the role of RS1 in retinal cell biology and X-linked retinoschisis, we have studied the interaction of wild-type and mutant RS1 with various carbohydrates coupled to agarose supports. RS1 bound efficiently to galactose-agarose and to a lesser extent lactose-agarose, but not agarose, N-acetylgalactosamine-agarose, N-acetylglucosamine-agarose, mannose-agarose, or heparin-agarose. RS1 cysteine mutants (C59S/C223S and C59S/C223S/C40S) which prevent disulfide-linked octamer formation exhibited little if any binding to galactose-agarose. The disease-causing R141H mutant bound galactose-agarose at levels similar to that of wild-type RS1, whereas the R141S mutant resulted in a marked reduction in the level of galactose-agarose binding. RS1 bound to galactose-agarose could be effectively displaced by incubation with isopropyl beta- d-1-thiogalactopyranoside (IPTG). This property was used as a basis to develop an efficient purification procedure. Anion exchange and galactose affinity chromatography was used to purify RS1 from the culture media of stably transformed Sf21 insect cells that express and secrete RS1. This cell expression and protein purification method should prove useful in the isolation of RS1 for detailed structure-function studies.

Structural similarities and functional diversity of eukaryotic discoidin-like domains

The discoidin domain is a approximately 150 amino acid motif common in both eukaryotic and prokaryotic proteins. It is found in a variety of extracellular, intracellular and transmembrane multidomain proteins characterized by a considerable functional diversity, mostly involved in developmental processes. The biological role of the domain depends on its interactions with different molecules, including growth factors, phospholipids and lipids, galactose or its derivatives, and collagen. The conservation of the motif, as well as the serious physiological consequences of discoidin domain disorders underscore the importance of the fold, while the ability to accommodate such an extraordinarily broad range of ligand molecules makes it a fascinating research target. In present review we characterize the distinctive features of discoidin domains and briefly outline the biological role of this module in various eukaryotic proteins.

Calcium-binding properties of SSP-5, the Streptococcus gordonii M5 receptor for salivary agglutinin

Streptococcus gordonii M5 expresses a lectin on its surface (SSP-5) which binds to human salivary agglutinin (SAG). This interaction requires sialic acid residues of SAG and divalent cations and may mediate the colonization of oral tissues by this organism. In this report, we show that the binding of SAG to SSP-5 requires calcium and that SSP-5 is a high-affinity calcium-binding protein. SAG-mediated aggregation of S. gordonii M5 was inhibited by 1 mM EDTA, and the restoration of aggregation occurred only upon the readdition of calcium. To ascertain the level at which calcium exerts its effects, the calcium-binding properties of SSP-5 were evaluated by using a 45Ca binding assay. In addition, a kinetic analysis of calcium binding was carried out by using fura2, a fluorescent calcium-binding dye. These analyses showed that SSP-5 is a high-affinity calcium-binding protein that binds 1 mol of calcium per mol of protein and has a dissociation constant of 0.45 +/- 0.2 microM. The calcium-binding capacity of SSP-5 was also calculated independently to be 1.0 +/- 0.2 mol of Ca per mol of SSP-5 by column chromatography on Sephadex G-25 equilibrated with 10 microM 45Ca. To localize the calcium binding site of SSP-5, a series of C-terminal deletion mutants were expressed in Escherichia coli and evaluated for calcium-binding activity. Deletion of the 250 C-terminal residues of SSP-5 had little effect on calcium binding. However, deletion of residues 1168 to 1250 resulted in the loss of calcium-binding activity, suggesting that this region is important for calcium binding by SSP-5.