Human sphingosine kinase: purification, molecular cloning and characterization of the native and recombinant enzymes.

Sphingosine 1-phosphate (S1P) is a novel lipid messenger that has important roles in a wide variety of mammalian cellular processes including growth, differentiation and death. Basal levels of S1P in mammalian cells are generally low, but can increase rapidly and transiently when cells are exposed to mitogenic agents and other stimuli. This increase is largely due to increased activity of sphingosine kinase (SK), the enzyme that catalyses its formation. In the current study we have purified, cloned and characterized the first human SK to obtain a better understanding of its biochemical activity and possible activation mechanisms. The enzyme was purified to homogeneity from human placenta using ammonium sulphate precipitation, anion-exchange chromatography, calmodulin-affinity chromatography and gel-filtration chromatography. This resulted in a purification of over 10(6)-fold from the original placenta extract. The enzyme was cloned and expressed in active form in both HEK-293T cells and Escherichia coli, and the recombinant E. coli-derived SK purified to homogeneity. To establish whether post-translational modifications lead to activation of human SK activity we characterized both the purified placental enzyme and the purified recombinant SK produced in E. coli, where such modifications would not occur. The premise for this study was that post-translational modifications are likely to cause conformational changes in the structure of SK, which may result in detectable changes in the physico-chemical or catalytic properties of the enzyme. Thus the enzymes were characterized with respect to substrate specificity and kinetics, inhibition kinetics and various other physico-chemical properties. In all cases, both the native and recombinant SKs displayed remarkably similar properties, indicating that post-translational modifications are not required for basal activity of human SK.

The solution structure of the cytokine-binding domain of the common beta-chain of the receptors for granulocyte-macrophage colony-stimulating factor, interleukin-3 and interleukin-5.

The haemopoietic cytokines, granulocyte-macrophage colony-stimulating factor, interleukin-3 and interleukin-5 bind to cell-surface receptors comprising ligand-specific alpha-chains and a shared beta-chain. The beta-chain is the critical signalling subunit of the receptor and its fourth domain not only plays a critical role in interactions with ligands, hence in receptor activation, but also contains residues whose mutation can lead to ligand-independent activation of the receptor. We have determined the NMR solution structure of the isolated human fourth domain of the beta-chain. The protein has a fibronectin type III fold with a well-defined hydrophobic core and is stabilised by an extensive network of pi-cation interactions involving Trp and Arg side-chains, including two Trp residues outside the highly conserved Trp-Ser-Xaa-Trp-Ser motif (where Xaa is any amino acid) that is found in many cytokine receptors. Most of the residues implicated in factor-independent mutants localise to the rigid core of the domain or the pi-cation stack. The loops between the B and C, and the F and G strands, that contain residues important for interactions with cytokines, lie adjacent at the membrane-distal end of the domain, consistent with their being involved cooperatively in binding cytokines. The elucidation of the structure of the cytokine-binding domain of the beta-chain provides insight into the cytokine-dependent and factor-independent activation of the receptor.