Effect of coat protein mutations in bacteriophage fd studied by sedimentation analysis

(a) Bacteriophage fd is a filamentous virus that has previously been well characterized. (b) Earlier work using point mutagenesis indicated that a lysine residue at position 48 in the major coat protein plays a crucial role in interacting with the DNA and governing the assembly into an intact virion. (c) In this study the sedimentation properties (sedimentation velocity and equilibrium) of wild-type fd and two mutants substituted at lysine-48 (K48Q and K48A) were compared. (d) Both mutants are similar to each other [M_r (19.5 ± 1.5) × 10⁶] but somewhat bigger than the wild-type [M_r (15.1 ± 1.5) × 10⁶]. The value for the wild-type is consistent with earlier published values. (e) By combining these data with sedimentation coefficient data, it is possible to compare the contour lengths and relative flexibilities of the mutants with those of the wild-type virion. (f) The mutants are shown hydrodynamically to have larger contour lengths (as also observed by electron microscopy): the ~20% difference in values obtained assuming rigid particle hydrodynamics with those obtained from electron microscopy is strongly suggestive of some difference in flexibility between the wild-type and mutants.