Morphology of the procarboxypeptidase A-S6 complex. A solution X-ray scattering study

Bovine pancreatic procarboxypeptidase A is secreted as a non-covalent association of three different proteins (pro CPA-S6). The free native subunits can be obtained by dissociation of the complex by dimethylmaleylation. Moreover, two specific binary complexes resulting from the high affinity of procarboxypeptidase A (subunit I) for its other two partners (subunits II and III) can also be obtained. In order to better understand the function of the association, an investigation of the morphology of the ternary complex by solution X-ray scattering has been carried out. The radii of gyration of all the molecular species have been obtained and the experimental results have been interpreted in terms of compact objects of simple shape. The various components correspond to globular particles as shown by the value of the ratio Rg/M1/3. This is confirmed by the moderate anisotropy of the simple geometric shapes determined using an assumed value of 0.3 g H2O/g protein for the hydration. The distances between the centres of gravity of pairs of species strongly suggest that the components are in the closest distance configuration or close to it. However, the binary complex I-III appears to be more open than the complex I-II. Finally, a model of the interaction between carboxpeptidase A and its activation peptide has been constructed by comparing the hypothetical geometric model of subunit I to the crystallographically determined structure of carboxypeptidase A.