Crystal structures of murine angiogenin-2 and -3 - probing 'structure - function' relationships amongst angiogenin homologues

摘要

Angiogenin (Ang) is a potent inducer of neovascularization. Point mutations in human Ang have been linked to cancer progression and two neurodegenerative diseases: amyotrophic lateral sclerosis and Parkinson's disease. Intensive structural and functional analyses of Ang have been paramount in assigning functions to this novel homologuc of bovine pancreatic RNasc A. However, inhibitor-binding studies with crystalline Ang (for designing potential anti-cancer drugs) have been hampered as a result of theinaccessibility of the active site. Experiments with the murine homologues of Ang have not only overcome the obvious practical limitations encountered when studying the role of a human protein in healthy individuals, but also the crystal structures of murine angiogenins (mAng and mAng-4) have revealed themselves to have greater potential for the visualization of small-molecule inhibitor binding at the active site. In the present study, we report the crystal structures of two more murine Ang paralogucs,mAng-2 and mAng-3, at 1.6 and 1.8 A resolution, respectively. These constitute the first crystal structures of an Ang with a zinc ion bound at the active site and provide some insight into the possible mode of inhibition of the ribonuclcolytic activityof the enzyme by these divalent cations. Both structures show that the residues forming the putative P_1, B, and B_2 subsitcs occupy positions similar to their counterparts in human Ang and arc likely to have conserved roles. However, a less obtrusive conformation of the C-tcrminal segment in mAng-3 and the presence of a sulfate ion in the B_1 subsitc of mAng-2 suggest that these proteins have the potential to be used for inhibitor-binding studies. We also discuss the biological relevance of the structural similarities and differences between the different Ang homologues.