Conformational states of a hydrophobic protein. The coat protein of fd bacteriophage.

The coat protein of fd bacteriophage has a short polypeptide chain of only 50 amino acid residues, containing a highly hydrophobic segment of 19 amino acids that is entirely devoid of ionic or other strongly polar amino acids. In the viral particle the protein exists as a closely packed array of alpha helices. It can be transformed to a monomeric randomly coiled polypeptide in very concentrated (greater than or equal to 7.3 M) guanidinium chloride. In anionic detergents or phospholipids the protein is dimeric, with a mixed conformation ("50% alpha"), the hydrophobic segment having a beta structure, whereas the two ends are predominantly alpha helical. In guanidinium chloride at concentrations of 6 M or less, and under other conditions in the absence of an anionic detergent or phospholipid, the protein forms an intractable polymer, with a beta-type conformation. If the protein is succinylated an oligomeric form of this structure (speculatively thought to be a soluble variety of a "beta barrel") can be obtained as a metastable state. The 50% alpha conformation, the beta oligomer, and the random coil can be interconverted reversibly, but formation of the beta polymer appears to be irreversible.