Many spherical viruses and some enzyme complexes form particles having icosahedral symmetry. The use of electron cryomicroscopy and single particle reconstruction has allowed us to visualize the 3-dimensional structures of this type of particle to 7-9 A resolution. This resolution is sufficient to resolve long alpha helices and recognize large beta sheets. In the past three years, we have determined 6 structures of icosahedral particles with sizes ranging from 600 -1250 A in this resolution range. They include herpesvirus capsid, rice dwarf virus, procapsid and mature forms of P22 bacteriophage, rotavirus and cytoplasmic polyhedrosis virus. With this experience, we have identified a number of possible improvements in the suite of software specific to the icosahedral particle reconstruction in order to improve the ease of its future maintenance and upgrade, the accuracy of determined structure, the ease of usage by biology end-users and the computational efficiency. In this proposal, we will have four specific goals.
The first aim i s to rewrite the major codes in C++, which includes the icosahedral particle center and orientation refinement and the Fourier Bessel reconstruction. They will be optimized for both shared and distributed computer platforms and will be written in modular form to make them easy to maintain and modify.
The second aim i s to improve the algorithms for particle orientation parameter refinement, contrast transfer function and B factor corrections and Fourier Bessel reconstruction respectively. We expect that these improved codes will lead to a more accurate structure determination of icosahedral particles.
The third aim i s to integrate the improved codes with a Python language binder together with a user-friendly graphical interface and to output relevant intermediate data processing results that would allow the users to decide if the refinement is heading to the correct structure convergence.
The fourth aim i s to document the codes adequately with a help menu and to disseminate them broadly and freely through the internet access. Before the software release, we will confirm their reliability and accuracy by testing with simulated and experimental data. The experimental data consist of structures that we have previously determined to subnanometer resolution and for which the crystal structures of some of their components are known.
