Actin/DNase I complex
Oryctolagus cuniculus (European rabbit)
Actin is one of the proteins that contribute to the contraction of muscles. It interacts with another protein, myosin (see PDB:1B7T), to form a complex that uses ATP to let muscles do their work. Actin itself is a normal, fairly small, protein. In its soluble form it is called G-actin. This is the form it takes when there is no salt present. If even small amount of salts are added, G-actin converts to another form, F-actin, that forms fibers. It was for this reason, that people had been unable to study the structure of actin in detail using X-ray crystallography, because it was impossible to obtain crystals of the protein. However, actin can form a complex with a protein, DNase I, entirely unrelated to its function. When this complex is formed, actin no longer polymerizes in the presence of salts. It was therefore possible to obtain crystals of this unnatural complex and determine the structures of the individual proteins. The structure of DNase I had been found before and this information was used to obtain the structure of actin as part of the complex.
The actin molecule consists of two domains that form a deep cleft and inside that cleft a molecule of adenosine triphosphate (ATP) is bound. A small, almost separate subdomain near the beginning of the actin sequence interacts very closely with the DNase molecule. Not too far away from this interaction site we see another subdomain of actin approaching the DNase molecule, although this interaction is not as tight as that from the first domain. It is somewhat peculiar to see the formation of such a complex, because naturally actin and DNase have very little to do with each other. Actin's function is to mobilize muscles, DNase's function is to cut DNA into pieces. This gives an example of the ability of proteins to interact without natural functional relationships between the interacting proteins.
Protein Data Bank (PDB)
Kabsch, W. Mannherz, H.G. Suck, D. Pai, E.F. Holmes, K.C.; "'Atomic structure of the actin:DNase I Complex'"; Nature; (1990) 347:37-44 PubMed:2395459.
author: Arno Paehler