Disulphide bonds can significantly stabilize the native structures of proteins. The effect is presumed to be due mainly to a decrease in the configurational chain entropy of the unfolded polypeptide. In phage T4 lysozyme, a disulphide-free enzyme, engineered disulphide mutants that crosslink residues 3-97, 9-164 and 21-142 are significantly more stable than the wild-type protein. To investigate the effect of multiple-disulphide bonds on protein stability, mutants were constructed in which two or three stabilizing disulphide bridges were combined in the same protein. Reversible thermal denaturation shows that the increase in melting temperature resulting from the individual disulphide bonds is approximately additive. The triple-disulphide variant unfolds at a temperature 23.4 degrees C higher than wild-type lysozyme. The results demonstrate that a combination of disulphide bonds, each of which contributes to stability, can achieve substantial overall improvement in the stability of a protein.

The overall folding of neutral protease from Bacillus subtilis has been predicted by computer-aided modelling, taking as a basis the known three-dimensional structure of thermolysin. As expected from the 50% similarity of sequence between the two proteins, the structure of B. subtilis protease is similar to that of thermolysin, including the two-domain topology and location of elements of regular secondary structure (helices and strands), whereas specific differences were predicted in loop regions. A protruding and loose loop predicted in B. subtilis has been detected also experimentally by a limited proteolysis approach. Incubation of B. subtilis protease at pH 9.0 for 24 h at room temperature with trypsin at 20: 1 ratio (by mass) leads to a specific and almost quantitative fission of the Arg214-Asn215 peptide bond located in a highly exposed, and thus probably flexible, loop of the protease. On the other hand, thermolysin was completely resistant to tryptic hydrolysis when reacted under identical conditions. The 'nicked' B. subtilis protease can be isolated by gel filtration chromatography at neutral pH, whereas the two constituting fragments 1 -214 and 21 5 -300 are separated under protein-denaturing conditions.Overall, these results indicate that the limited proteolysis approach can pinpoint a peculiar difference in surface structure between the two similar protein molecules of B. subtilis neutral protease and thermolysin and emphasize the potential use of proteolytic enzymes as structural probes of globular proteins. subtilis [lo] are known. These sequence data, obtained by direct protein sequencing or by analyzing the corresponding cDNA, clearly established that these proteases show high degrees of sequence similarities, this being higher among thennostable proteases derived from thermophilic microorganisms, as well as among thermolabile ones from mesophilic sources. Moreover, the crystal structure of thermolysin [I 1 -141 and of the mesophilic neutral protease from B. cereus [15] have been reported. Thus, this ample set of well characterized (thermophilic and mesophilic) neutral proteases constitute an excellent model system for addressing problems of structure/ functionlstability relationships. As a matter of fact, initial studies of protein engineering experiments on neutral proteases by site-directed mutagenesis have already appeared [16, 171. Correspondence to A. Fontana, Department of Organic Chemistry, Via Marzolo 1, 1-35131 Padua, Italy.Abbreviations. Gdn In the ambit of a research program aimed to prepare and study mutants of B. subtilis neutral protease (NP) [17], it was of importance to develop a model of this protease in order to design interesting mutants and to interpret the possible effects of site-specific mutagenesis in terms of conformational details and interactions within the protein molecule. In this paper we report a proposed model of NP predicted on the basis of the known three-dimensional structure of TLN [14] with the aid of a graphics display. As expected from the 50% seq...

This study is focused on the characterization of the interaction of the amphiphilic peptide bombolitin III (from the bumblebee Megabombus pennsylvanicus) with phospholipid monolayers and vesicles. It is shown that due to the amphiphilic character of its alpha-helical conformation this water-soluble peptide is able to interact in an ordered fashion with phospholipid organized structures. Depending on the temperature, the subphase, and the particular phosphatidylcholine used, the mixed peptide-phospholipid monolayers can be homogeneous or display phase separation. This behavior was observed by means of the Langmuir film balance technique, coupled with an epifluorescence microscope. In well-defined conditions it is possible to visualize the formation of phase-separated peptide domains at the air-water interface and to study the effect of their presence on the organization of the lipid. The action of phospholipase A2 at the lipid-peptide interface was also followed by means of fluorescence microscopy: some evidence that the enzyme preferentially hydrolyzes the phospholipid that is in contact with the peptide is presented. Furthermore, the presence of bombolitin III in L-alpha-DLPC monolayers causes an increase in the initial speed of degradation with phospholipase A2. These results are in agreement with previous findings that show that the bombolitins are activators in vitro of phospholipase A2. Experiments were also performed with peptide fragments corresponding to the alpha-helical sequences of the protein uteroglobin: despite some amphiphilic character, these peptides do not interact strongly with phospholipid monolayers. Only one of these peptides (corresponding to the helix 4-14 in uteroglobin) is adsorbed in the monolayer in a similar fashion to bombolitin III but does not cause an increase in the activity of phospholipase A2.

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