Crystal Structure of the Rad3/XPD Regulatory Domain of Ssl1/p44 [Protein Structure and Folding]

February 13th, 2015 by Kim, J., Saint–Andre, C., Lim, H., Hwang, C.–S., Egly, J. M., Cho, Y.

The Ssl1/p44 subunit is a core component of the yeast/mammalian general transcription factor TFIIH, which is involved in transcription and DNA repair. Ssl1/p44 binds to and stimulates the Rad3/XPD helicase activity of TFIIH. To understand the helicase-stimulatory mechanism of Ssl1/p44, we determined the crystal structure of the N-terminal regulatory domain of Ssl1 from Saccharomyces cerevisiae. Ssl1 forms a von Willebrand factor A (VWA) fold in which a central six-stranded β-sheet is sandwiched between three α helices on both sides. Structural and biochemical analyses of Ssl1/p44 revealed that the β4-α5 loop, which is frequently found at the interface between VWA family proteins and cellular counterparts is critical for the stimulation of Rad3/XPD. Yeast genetics analyses showed that double mutation of Leu239 and Ser240 in the β4-α5 loop of Ssl1 leads to lethality of a yeast strain, demonstrating the importance of the Rad3-Ssl1 interactions to cell viability. Here, we provide a structural model for the Rad3/XPD-Ssl1/p44 complex and insights into how the binding of Ssl1/p44 contributes to the helicase activity of Rad3/XPD and cell viability.