Crystal structures of the complexes of secretory glycoproteins with designed peptides.

Department of Biophysics, All India Institute of Medical Sciences, New delhi.
○A.S. Ethayathulla Devendra B. Srivastava Janesh Kumar Rishi K. Somvanshi Nagendra Singh Sujata Sharma Sharmistha Dey Asha Bhushan T. P. Singh

Crystal structures of a new family of glycoproteins (SPX-40) that act normally as protective signalling factor and determine which cells are to survive the drastic tissue remodeling during involution, have been solved. This protein from Sheep (SPS-40), Porcine (SPP-40) and Goat (SPG-40) was isolated from dry secretions of mammary glands. Crystal structures of native as well as the complexes of SPX-40 with designed peptides Trp-Pro-Trp (WPW) and Trp-His-Trp (WHW) were determined. The structures revealed a topology with beta/alpha domain having the Triose-Phosphate Isomerase (TIM) barrel fold in the core and a small alpha + beta domain. These structures are similar to chitinases but lack chitin binding site due to structural differences and a loss of chitin hydrolyzing capability due to point mutations in the active site. Apparently, these proteins bind to cell surface receptors and protect the viable epithelial cells from extensive tissue remodeling during involution. These structures of complexes revealed that the interactions are predominantly hydrophobic interactions. The key protein residues involved in these interactions were W78, Q83, D186, W191, R192, P260, E269 and K270. The binding studies with fluorescence spectroscopy indicated a strong affinity between SPX-40 and the ligand. The real time kinetics measurement carried out using BIAcore-2000 showed a dissociation of 10-7 M. The complexes with designed peptides have thrown a lot of light towards understanding their functions and suggested a good scope for the design of specific inhibitors of these proteins that might be eventually exploited as therapeutic agents against breast cancer.