Three-dimensional structure of a cloning vector. X-ray diffraction studies of filamentous bacteriophage M13 at 7 A resolution.

Filamentous bacteriophage M13 is a single-stranded DNA phage about 65 A in diameter and 9300 A long. X-ray diffraction studies of magnetically oriented fibers of native, mercury and iodine-labeled phage particles have been used to determine the arrangement of the major coat protein, the gene 8 product, in the virion. The coat protein is made up of a single gently curving alpha-helix extending from approximately Pro6 to near the carboxyl terminus. The axis of the alpha-helix is tilted about 20 degrees from the viral axis and wraps around the axis in a right-handed helical sense. The surface of the virus is made up largely of polar residues in the amino-terminal half of the protein including the segment of alpha-helix extending from Pro6 to Tyr24. The interior surface of the protein coat faces the DNA and consists of an amphipathic helical segment extending from Thr36 to Ser50. The alpha-helices form a tightly packed 15 to 20 A thick cylindrical coat around the DNA. This structural model provides insight into the potential sites for incorporating foreign protein domains that may act as functional binding sites on the surface of M13.