THE XPB SUBUNIT OF REPAIR TRANSCRIPTION FACTOR TFIIH DIRECTLY INTERACTS WITH SUG1, A SUBUNIT OF THE 26S PROTEASOME AND PUTATIVE TRANSCRIPTION FACTOR/

Mutations in the basal transcription initiation/DNA repair factor TFII H are responsible for three human disorders: xeroderma pigmentosum (XP ), Cockayne syndrome (CS) and trichothiodystrophy (TTD). The non-repai r features of CS and TTD are thought to be due to a partial inactivati on of the transcription function of the complex. To search for protein s whose interaction with TFIIH subunits is disturbed by mutations in p atients we used the yeast two-hybrid system and report the isolation o f a novel XPB interacting protein, SUG1. The interaction was validated in vivo and in vitro in the following manner. (i) SUG1 interacts with XPB but not with the other core TFIIH subunits in the two-hybrid assa y. (ii) Physical interaction is observed in a baculo-virus co-expressi on system. (iii) In fibroblasts under non-overexpression conditions a portion of SUG1 is bound to the TFIIH holocomplex as deduced from co-p urification, immunopurification and nickel-chelate affinity chromatogr aphy using functional tagged TFIIH. Furthermore, overexpression of SUG 1 in normal fibroblasts induced arrest of transcription and a chromati n collapse in vivo. Interestingly, the interaction was diminished with a mutant form of XPB, thus providing a potential link with the clinic al features of XP-B patients. Since SUG1 is an integral component of t he 26S proteasome and may be part of the mediator, our findings disclo se a SUG1-dependent link between TFIIH and the cellular machinery invo lved in protein remodelling/degradation.