Functions as a component of the DNA-binding general transcription factor complex TFIID. Binding of TFIID to a promoter (with or without TATA element) is the initial step in pre-initiation complex (PIC) formation. TFIID plays a key role in the regulation of gene expression by RNA polymerase II through different activities such as transcription activator interaction, core promoter recognition and selectivity, TFIIA and TFIIB interaction, chromatin modification (histone acetylation by TAF1), facilitation of DNA opening and initiation of transcription 