The kinase activity of P-TEFb is inhibited when the factor is in the 7SK snRNP. P-TEFb can be released from the 7SK snRNP by either the HIV transactivator Tat or the bromodomain containing protein BRD4. This release leads to a conformational change in 7SK RNA and the ejection of HEXIM.
The binding of P-TEFb to the 7SK snRNP complex inhibits its kinase activity. Conversely, the release of P-TEFb from this association by Tat or BRD4 triggers a conformational change in 7SK RNA and leads to the expulsion of HEXIM protein.