Kinetic parameters of the translocation of bacteriophage T4 gene 41 protein helicase on single-stranded DNA.

The bacteriophage T4 gene 41 helicase protein (gp41) carries a single-stranded DNA-dependent ATPase activity that is essential to its helicase activity. This ATPase activity can be stimulated by a wide variety of single-stranded DNA cofactors, including homo-oligomers and homopolymers 8 to approximately 10,000 nucleotide residues in length, and by natural single-stranded DNA, such as bacteriophage M13 DNA. The steady-state ATPase activity of gp41 on single-stranded homopolymeric cofactors is dependent on the length of the cofactor, in that the kinetic parameters Vmax and K(act) (or KmDNA) have a characteristic length dependence. Vmax values for different DNA lengths show a hyperbolic dependence on DNA length, while K(act) values are independent of DNA lengths exceeding approximately 20 nucleotide residues. Use of the detailed theoretical analysis developed in the preceding paper reveals that: (1) these results support the earlier proposal that gp41 translocates on single-stranded DNA in an ATP-dependent manner; (2) translocation is undirectional; (3) translocation is processive to an extent that depends on the base composition of the DNA employed, with the average distance translocated per binding event ranging from 60 to 700 nucleotide residues; and (4) the detailed translocation mechanism of gp41 includes an obligatory slow step before or after the ATP-driven translocation process. Defined lengths of natural and homopolymer single-stranded DNA have also been created as gaps of known length distribution between clusters of gene 32 protein (gp32) bound along long single-stranded DNA molecules. ATPase data obtained with cofactors of this type also show unidirectional ATP-driven translocation of gp41 on both natural and homopolymeric single-stranded DNA. Direct binding studies of gp41 to short dT oligomers reveal two further features of the interaction of gp41 to single-stranded DNA: (1) nucleoside triphosphate binding is necessary for the formation of stable gp41-ssDNA complexes; and (2) the DNA binding site size of gp41 is between 12 and 20 nucleotide residues per protein monomer. Possible translocation mechanisms for gp41 are discussed within the context of these results.