High-speed replication of chromosomal DNA requires the DNA polymerase to be attached to a sliding clamp (known as proliferating cell nuclear antigen, or PCNA, in eukaryotes) that prevents the polymerase from falling off DNA. In all cells and in some viruses, the clamp is a ring-shaped protein complex that encircles DNA, forming a sliding platform on which DNA polymerases and other proteins that move along DNA are assembled. Sliding clamps play a part in DNA replication, DNA repair, cell cycle control and modification of chromatin structure and defects in several clamp-associated factors are associated with cancer and other disorders caused by abnormalities in DNA replication and repair. Sliding clamps from different branches of life have different subunit stoichiometry (they are dimers in bacteria and trimers in eukarya, archaea and bacteriophages. Nevertheless, their structures are remarkably similar. The conserved structure is an elegant symmetrical elaboration of a simple β-α-β motif, repeated 12 times around a circle. The circular geometry is broken when the clamp is opened for loading onto DNA, but the elegance is retained during the loading step as the clamp assumes a helical symmetry that reflects the helical symmetry of DNA. Here you can see a zoomed detail of the x-ray structure of T4 Bacteriophage clamp loader bound to the T4 clamp and a DNA template (PDB code: 8UH7)

#molecularart ... #phage ... #T4 ... #clamp ... #replication ... #loader ... #xray

Structure rendered with @proteinimaging and depicted with @corelphotopaint