The solution structure of the DNA-binding domain of Skn-1.

Skn-1 is a maternally expressed transcription factor that specifies the fate of certain blastomeres early in the development of Caenorhabditis elegans. It has been reported that the DNA-binding domain is a molten globule and that the structure cannot be defined because there are no long-range nuclear Overhauser effects (NOEs). Working with short Skn domain fragments and using 13C-labeled proteins, we have been able to identify 28 long-range NOEs that establish a tertiary fold for the Skn domain. The internal region of the Skn domain consists of three stable helices and one conformationally labile helix organized into a nascent helix-turn-helix-turn-helix-turn-helix motif. The N and C termini of the Skn domain are unstructured and emerge from the same end of the folded domain. This structure is consistent with biochemical data on binding of the Skn domain to DNA, which shows that the N and C termini bind in the adjacent minor and major grooves from the same face of the DNA helix. The NMR solution structure of the Skn domain should be useful for developing a complete understanding of the DNA recognition event, including any conformational changes that take place upon binding.