Interactions in solution of a large oligomeric protein

Up to now, systematic studies on protein interactions in solution have been (mostly) restricted to small molecular weight model proteins like lysozyme , BPTI or gamma-crystallins. Those studies involving a combination of techn iques (osmotic pressure, light scattering, small-angle X-ray scattering, et c.) led to an interpretation of the results in terms of interaction potenti als, the parameters of which can be related in a semi-quantitative way to v an der Waals forces and particle charges. We have undertaken an X-ray scatt ering study to extend the interaction potential analysis to the case of a l arge size oligomeric protein, aspartate transcarbamylase from E. coli. This heterododecamer comprises two trimers of catalytic chains and three dimers of regulatory chains for a total molecular weight of 306 kDA. It is a slig htly acidic protein (pI = 5.9). The main thermodynamic and chemical paramet ers were varied: temperature, protein concentration, pH, salt nature and co ncentration. Moreover, we took advantage of the large molecular weight of A TCase to study the effect of polyethylene glycols. The results are compared to those reached in the case of small proteins. (C) 1999 Elsevier Science B.V. All rights reserved.