The mammalian TFIID protein is present in two functionally distinct complexes.

The TFIID activity recognizes a TATA-box element and supports formation of an initiation complex containing RNA polymerase II. Antisera specific for the 38-kD human TFIID protein were used to determine whether this protein cofractionated with the TFIID activity. Surprisingly, the TFIID activity in HeLa whole-cell extracts was resolved into two different size complexes, one of 300 kD and one of greater than 700 kD. Cofractionation studies suggest that both complexes contain the 38-kD protein; thus, this component of the large complexes is probably responsible for recognition of the TATA sequence and interaction with the other general transcription factors in formation of the initiation complex. Interestingly, in contrast to the TFIID activity characterized previously, the 300-kD form of TFIID activity, B-TFIID, does not support stimulation of transcription by factors containing acidic or glutamine-rich activating motifs. We propose that the functional and physical differences between these two forms of TFIID activity are caused by differences in the protein composition of the TFIID complexes of which the 38-kD hTFIID protein is an integral part.