Grafting of a Calcium-Binding Loop of Thermolysin to Bacillus subtilis Neutral Protease

The surface loop which in the Bacillus subtilis neutral protease (NP) extends from amino acid residue 188 to residue 194 was replaced, by site-directed mutagenesis, with the 10-residue segment which in the homologous polypeptide chain of thermolysin (TLN) binds calcium-4 [Matthews, B. W., Weaver, L. H., & Kester, W. R. (1974) J. Biol. Chem. 249, 8030-8044]. The mutant NP was isolated to homogeneity, and its structural, functional, calcium-binding, and stability properties were investigated. Proteolytic fragmentation with Staphylococcus aureus V8 protease of mutant NP was used to isolate and analyze the protein fragment encompassing the site of mutation, unambiguously establishing the effective insertion of the new 10-residue segment. Atomic absorption measurements allowed us to demonstrate that mutant NP binds three calcium ions instead of the two ions bound to wild-type NP, showing that indeed the chain segment grafted from TLN to NP maintains its calcium-binding properties. The mutant NP showed kinetic parameters essentially similar to those of the wild-type NP with Z-Phe-Leu-Ala-OH as substrate. The enzyme inactivation of mutant vs wild-type NP was studied as a function of free [Ca2+]. It was found that mutant NP was much less stable than the wild-type NP when enzyme solutions were dialyzed at neutral pH in the presence of [Ca2+] below 10(-3) M. On the other hand, the kinetic thermal stability to irreversible inactivation of mutant NP, when measured in the presence of 0.1 M CaCl2, was found to be increased about 2-fold over that of the wild-type NP. Thus, modulation of enzyme stability by free [Ca2+] in mutant NP correlates with similar findings previously reported for thermolysin. Overall, the results obtained indicate that protein engineering experiments can be used to prepare hybrid proteins on the basis of sequence and function analysis of homologous protein molecules and show the feasibility of engineering metal ion binding sites into proteins.