X-ray crystallographic studies on the structure and interactions of the profilin:actin complex
Actin is one of the 2 main proteins in muscle, and is also involved in many non-muscle motile processes. When actin is extracted from non-muscle cells, it is often found as a 1:1 complex with profilin which reduces its tendency to polymerise. The control of this interaction is important and necessary for the production of some labile, filamentous actin containing structures, and for some non-muscle motile events. The profilin:actin complex was originally crystallised as an unknown inhibitor of DNase I and, although actin alone will form a tight 1:1 complex with Dnase I, no physiological explanation has been found for these interactions. The profilin:actin crystals crystallise in the orthorhombic space group P212121, with cell dimensions of a = 3.85 nm, b = 7.23 nm and c &61 18.74 nm, and have one molecule of each protein per asymmetric unit. The crystals diffract to high resolution, but are susceptible to large changes in the cell dimensions; in particular the c-axis will shrink to as little as 16.5 nm. Interaction of heavy metal salts with the crystals is one of the factors which produce large changes in the cell dimensions, so much work had been done to stabilise the crystals to make isomorphous heavy atom derivatives. At the start of this project, 3 fairly isomorphous heavy atom derivatives had already been discovered and data to high resolution collected on 2 of them. However, the data at high resolution contained large errors due to absorption and the derivatives were not totally isomorphous. Thus I set out to discover some new heavy atom derivatives and also to try to improve the use of the existing data so that the structure of the protein complex could be determined at high resolution. Two new derivatives were discovered, one utilising a modified ATP molecule to introduce a reactive group into the complex and both using the mercury salt p-hydroxy mercuribenzoate. Precession film, diffractometer and oscillation film data were collected on these two derivatives as well as recollecting poor data and missing data for the other derivatives. Some of the previously collected data were reprocessed and all the derivatives, at all resolution limits, from all sources were reexamined to find the best possible set of heavy atom solutions and thus produce the best set of phases and the most reliable electron density map. The result of this work was an electron density map which was much more interpretable in terms of protein structure. Previously, very little regular secondary structure could be seen but in the new map α-helices, β-sheets and the ATP molecule could all be found. Once regular secondary structure can be found, modifications can be made to the phases and the results can be observed to improve, or otherwise, the map. The structure of the profilin:actin complex is now being built and even if the complete atomic coordinates can not be found, there exists a rigorous framework on which to base further work.