Molecular biology

Molecular cloning of human G-CSF has yielded purified material (rhG-CSF) that is biologically active in colony assays when used at a concentration of 5— 10 pM. The mRNA encodes a 30 amino acid hydrophobic leader sequence that is characteristic of secreted proteins. A typical signal sequence cleavage site is followed by 174 amino acids, giving an M, of 18 627: two disulfide bonds stabilize the molecule. Although native G-CSF is glycosylated, the sugar residues do not appear to be important for biological activity; recombinant material produced by Escherichia colt is apparently as active in vitro and in vivo as the native glycosylated material.

X-ray crystallography has shown that nonglycosy-lated G-CSF has four antiparallel bundles with crossing angles of 18°, a short 3-10 helix immediately after the first disulfide bond, and two long loops and one short loop connecting the helical core bundle. Although there is a structural relationship between G-CSF and other cytokines such as IL-2, IL.-3, I1.-6, erythropoietin and GM-CSF, it is most closely related to human growth hormone in terms of the four helices and folding and positioning of the helices within the core structure.

The gene for human G-CSF is single copy, does not share homology with other CSFs, and has been localized to the q21-22 region of chromosome 17 (unlike M-CSF, GM-CSF, IL-3, IL-4 and II.-5, which are found as a linked gene cluster on chromosome 5). This region of chromosome 17 is involved in the translocation between chromosomes 15 and that is associated with acute promyelocyte leukemia.

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