Three-dimensional structures of proteins determined by X-ray crystallography provide important clues on the molecular mechanism of protein folding, which is the last step of gene expression. The present status on the native structures revealed from the accumulated data is reviewed. Furthermore, the stability of a native conformation is dependent on the environment of a protein molecule such as temperature, and denaturation from a native state occurs as the transition of two states, the native and denatured states. The denaturation phenomenon is analyzed in terms of intermolecular interactions. The understanding of the protein architecture may be essential for the progress of the protein engineering.
Recent development of genetic recombination technique for study of the function-structure relationships of protein is reviewed. After elucidation of the nature of genetic message, fundamentals in the technique, both creating recombinant DNA molecules and the isolation of cloned genes are explained and some of the in vitro mutagenesis are mentioned for futher use of the technique with some examples.
The site-directed mutagenesis opens the doors to investigations of the fundamental nature of proteins and to additions of new valuable characters to proteins. In such circumstances, it is desired to speed up the X-ray structure analyses of protein molecules. Area detectors are expected as one of the promising outcomes of recent technical developments and it can collect diffraction intensities many times more rapidly than current instruments. One can design a new protein from the many results of these protein structure determinations and also of the closely related protein structures. [J. Cryst. Soc. Jpn. 29, 27 (1987) ] .
The protein engineering is expected to bear fruitful products from the industrial and scientific aspects. It is entirely based on the three dimensional structure of proteins. The recent studies on the protein structures by X-ray suggest how to modify the protein by replacement of an amino acid residue into the other. The useful protein which has not been found in the natural world will be also produced by the protein engineering, inspecting the protein structure carefully. [J. Cryst. Soc. Jpn. 29, 38 (1987) ] .
Firstly, the crystal structures of native ribonuclease T1 complexes with 2'-GMP, an inhibitor, and with 3'-GMP, a product, at pH 5.7 and 4.0 are described in detail, and then the cloning and expression of ribonuclease T1 gene by genetic engineering are briefly reviewed. Finally, I will discuss the structure-function relationships, especially between ribonuclease T1 with correct amino acid sequence and that with incorrect and old sequence, and also between native ribonuclease T1 and mutants replaced some amino acid residues at active center. [J. Cryst. Soc. Jpn. 29, 44 (1987) ] .