Electrostatic potential distribution of the gene V protein from Ff phage facilitates cooperative DNA binding: A model of the GVP‐ssDNA complex

The crystal structure of the gene V protein (GVP) from the Ff filamentous phages (M13, f1, fd) has been solved for the wild‐type and two mutant (Y41F and Y41H) proteins at high resolution. The Y41H mutant crystal structure revealed crystal packing interactions, which suggested a plausible scheme for constructing the polymeric protein shell of the GVP‐single‐stranded DNA (ssDNA) complex (Guan Y, et al., 1994, Biochemistry 33:7768–7778). The electrostatic potentials of the isolated and the cooperatively formed protein shell have been calculated using the program GRASP and they revealed a highly asymmetric pattern of the electrostatic charge distribution. The inner surface of the putative DNA‐binding channel is positively charged, whereas the opposite outer surface is nearly neutral. The electrostatic calculation further demonstrated that the formation of the helical protein shell enhanced the asymmetry of the electrostatic distribution. A model of the GVP‐ssDNA complex with the n = 4 DNA‐binding mode could be built with only minor conformational perturbation to the GVP protein shell. The model is consistent with existing biochemical and biophysical data and provides clues to the properties of GVP, including the high cooperativity of the protein binding to ssDNA. The two antiparallel ssDNA strands form a helical ribbon with the sugar‐phosphate backbones at the middle and the bases pointing away from each other. The bases are stacked and the Phe 73 residue is intercalated between two bases. The optimum binding to a tetranucleotide unit requires the participation of four GVP dimers, which may explain the cooperativity of the GVP binding to DNA.