Identification of SAF-2, a novel siglec expressed on eosinophils, mast cells, and basophils

BACKGROUND

Eosinophils, basophils, and mast cells are believed to be the central tenet cells in allergic conditions including allergic rhinitis, asthma, and eczema. The molecular mechanisms underlying the recruitment of these cells to sites of allergic inflammation are poorly understood.

OBJECTIVES

Our aim was to identify a common adhesion molecule that could potentially be responsible for mediating the recruitment of the allergic cell types to the lungs and other sites of allergy.

METHODS

We have cloned a sialoadhesin molecule from a human eosinophil library with the use of expressed sequence tag technology and characterized its expression on allergic cells by the use of flow cytometry and specific mAbs.

RESULTS

With the use of expressed sequence tag sequencing, we have identified a novel siglec molecule, SAF-2. SAF-2 has homology with other sialoadhesin family members (CD33 and siglec-5) and belongs to a subgroup of the Ig superfamily. SAF-2 is a 431-amino acid protein composed of 3 Ig domains with a 358-amino acid extracellular domain and a 47-amino acid tail. SAF-2 is highly restricted to eosinophils, basophils, and mast cells. Antibodies to SAF-2 do not modulate Ca(++) mobilization or chemotaxis of human eosinophils induced by eotaxin.

CONCLUSION

SAF-2 is a highly restricted sialoadhesin molecule, which may be useful in the detection and/or modulation of allergic cells.

Expression in Escherichia coli, refolding, and purification of human procathepsin K, an osteoclast-specific protease

We have constructed and optimized a high yielding Escherichia coli expression system to produce glycosylation-free human procathepsin K and have developed conditions for refolding this enzyme. Recombinant human procathepsin K (EC 3.4.22.38) was expressed in E. coli, refolded from inclusion bodies, and further purified by Superdex 75 size-exclusion chromatography. Purified procathepsin K had a [MH]+ of 35,063 Da which is in agreement with the predicted mass of the construct. Amino-terminal sequence analysis matched the predicted sequence with no secondary sequence detected. Purified procathepsin K activated under autocatalytic conditions to a final specific activity of 23 micromol 7-amido-4-methylcoumarin liberated/min/mg of enzyme using the fluorescent peptide substrate benzyloxycarbonyl-phenylalanine-arginine-7-amido-4-methylcoumarin. This expression and refolding procedure yielded 50 mg of purified, glycosylation-free human procathepsin K from 1 liter of E. coli cell culture and enabled the determination of the structure of human procathepsin K at 2.6 A resolution.