CTAP-IIIdes10: IMPLICATIONS OF AMINO TERMINAL RESIDUES ON CHEMOTACTIC ACTIVITY AND RECEPTOR ACTIVATION. Michael G. Malkowski¹, Jerome B. Lazar², Paul H. Johnson², and Brian F.P. Edwards¹. Department of Biochemistry, Wayne State University, Detroit, MI 48201¹ and Department of Molecular Biology, SRI International, Menlo Park, CA 94025².

CTAP-IIIdesl0, which was generated by recombinant methods, with Met-ll replaced by leucine, has 10 fewer amino terminal residues than Connective Tissue Activating Protein-III (CTAP-III) but 5 more than Neutrophil Activating Peptide-2 (NAP-2). CTAP-III (85 residues) and NAP-2 (70 residues), members of the CXC chemokine family, are carboxy terminal fragments of Platelet Basic Protein (PBP, 94 residues) which arise from cleavage by monocyte-derived proteases. NAP-2 has powerful neutrophil stimulating effects involved in inflammation, whereas the larger precursors, PBP and CTAP-III, are inactive. NAP-2 behaves like a typical chemotactic receptor agonist inducing a rise in cytosolic calcium, chemotaxis, and exocytosis at concentrations between 0.3 and 10nM. PBP, CTAP-I1I, and NAP-2 all contain the highly conserved Glu-Leu-Arg (ELR) region that is critical for receptor binding. The longer isoforms, PBP and CTAP-III, are inactive toward receptors at physiological concentrations. We have proposed that the extended amino terminus folds back to interact with the ELR region and block access to the receptor (Malkowski et al., J. Biol. Chem. 270: 7077 7087 (1995)). We have determined the structure of CTAP-IIIdes10 in a monoclinic space group (P2₁), using the molecular replacement method, with a tetramer in the asymmetric unit, to a final R factor of 0.196 (R_free=0.251) for 2 sigma data from 7.0 to 1.75Å resolution. Clear, continuous density is seen for the extended amino terminus which does indeed fold back through a type-II turn and interact with the ELR region.

This work was supported in part by NIH grant GM33192. MGM was supported by NIH training grant T32 HL07602.