What is it about?

The nominal size limit for protein structure determination by direct methods (probability theory) is 1000 non-hydrogen atoms. The presences of a heavy atom enabled structure determination from the native diffraction data and probability theory as encoded in SIR2004. No anomalous difference data nor prior knowledge of the position of the heavy atom were used in structure determination. The structure of bacteriophage P22 was unknown and had defied structure determination for over 20 years. Sequential additions of glycerol to the reservoir after crystallization drop setup increased the volume of the crystal eight-fold and thereby led to the required 1.05 Angstrom data.

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Why is it important?

We call this approach augmented direct methods. This is important because it shows how the purposeful addition of heavy atoms can extend the application of direct methods to larger proteins. This approach is an option when anomalous diffraction data differences and good molecular replacement models are absent. The results of a series of structure determination trials with truncated diffraction data demonstrated the weakness of the R-factor for detecting correct structures at resolution limits below 1.3 Angstroms. The figure that summarizes these trials is very useful in teaching, especially when emphasizing that many phase sets corresponding to random atom positions can lead to low R-factors and these R-factors decline as the resolution cutoff declines.

Perspectives

This paper also highlights the successful application of the SIR software to a challenging unknown protein by a lab that is not the developer of this or any other direct methods software. The SIR software is not as well known to the biological crystallography community as some of the competing software. The developers of SIR are located at the CNR Institute of Crystallography at the University of Bari in Italy and are led by Professor Carmelo Giacovazzo.

Professor Blaine Mooers
University of Oklahoma Health Sciences Center

Read the Original

This page is a summary of: Extension to 2268 atoms of direct methods in theab initiodetermination of the unknown structure of bacteriophage P22 lysozyme, Acta Crystallographica Section D Biological Crystallography, January 2006, International Union of Crystallography,
DOI: 10.1107/s0907444905037212.
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