Chemical Shift Mapped DNA-Binding Sites and 15N Relaxation Analysis of the C-Terminal KH Domain of Heterogeneous Nuclear Ribonucleoprotein K

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Abstract
The K homology (KH) motif is one of the major classes of nucleic acid binding proteins. Some members of this family have been shown to interact with DNA while others have RNA targets. There have been no reports containing direct experimental evidence regarding the nature of KH module−DNA interaction. In this study, the interaction of the C-terminal KH domain of heterogeneous nuclear ribonucleoprotein K (KH3) with it's cognate single-stranded DNA (ssDNA) are investigated. Chemical shift perturbation mapping indicates that the first two helices, the conserved GxxG loop, β1, and β2, are the primary regions involved in DNA binding for KH3. The nature of the KH3−ssDNA interaction is further illuminated by a comparison of backbone 15N relaxation data for the bound and unbound KH3. Relaxation data are also used to confirm that the backbone of wild-type KH3 is structurally identical to that of the G26R mutant KH3, which was previously published. Amide proton exchange experiments indicate that the two helices involved in DNA binding are less stable than other regions of secondary structure and that a large portion of KH3 backbone amide hydrogens are protected in some manner upon ssDNA binding. The major backbone dynamics features of KH3 are similar to those of the structurally comparable human papillomavirus-31 E2 DNA binding domain. Secondary structure information for ssDNA-bound wild-type KH3 is also presented and shows that binding results in no global changes in the protein fold.