AN OPEN-STATE OF [[beta]]-ACTIN AND THE SOLID-STATE TRANSFORMATIONS OF PROFILIN:[[club]]-ACTIN CRYSTALS John K. Chik (NIH), Uno Lindberg, Stockholm U. and C.E. Schutt, Princeton U.

The 2.65Å structure of an "open-state" of bovine [[club]]-actin reveals many differences to our previously solved "tight-state" structure (C.E. Schutt et al., Nature, 365:810-816 (1993)) such as solvent accessible ATP phosphates. It was also possible to estimate the energy difference between these two states using osmotic pressure. The conformation of [[club]]-actin complexed with profilin is sensitive to the environment surrounding the crystal. Profilin:[[club]]-actin crystals normally grown in potassium phosphate were sensitive enough to solution condition so as to make them unsuitable for isomorphous heavy-atom methods (Schutt et al., J. Mol. Biol., 209:735-746 (1989)). Transferring the 1.8M potassium phosphate grown crystals into 3.2M ammonium sulphate yielded better diffracting crystals that were also amenable to heavy-atom methods. This transition from potassium phosphate, open-state, to ammonium sulphate, tight-state, buffer caused the c-dimension of the unit-cell to shrink from 185.7Å to 171.9Å without deleterious damage to the crystal. These tight-state crystals lead to the 2.55Å structure of the complex (C.E. Schutt et al., Nature, 365:810-816 (1993)). The structure of the open-state complex was solved using molecular replacement (R=20.1% and Rfree=32.8% from 8.00-2.65Å). The open- to tight-state transition buries the solvent exposed ATP phosphates, closes the cleft between the domains and rotates sub-domain 2 by 14.7deg.. Also, as seen in tight-state crystals, open-state [[club]]-actins form extensive contacts to symmetry related actins along the 2₁ axis parallel to the b-direction. This "ribbon" feature is maintained in the open- to tight-state transition. Using osmotic pressure, it was possible change the unit-cell dimensions to a similar degree as seen in the open- to tight-state transition and thus, by inference, provides an experimental estimate of the energy difference between the states. The estimated difference is only a fraction of thermal energy and is consistent with the observed sensitivity of the crystals.