Figure 6.

Formation and composition of 7SK snRNP. Following the folding of 7SK RNA into a four-stem loop structure, MePCE and LARP7 bind and protect its 5’ and 3’ ends, respectively, from catalytic degradation. One mechanism of protection involves capping of the 5’ end of 7SK RNA by MePCE (depicted as a black dot). The stable 7SK snRNP core then binds dimers of HEXIM1 which exposes their P-TEFb binding domains. Subsequently, HEXIM1 binds activated P-TEFb (CDK9 phosphorylated on Thr186, green dot) and this inhibits its kinase activity. During transcriptional activation, P-TEFb is released and 7SK snRNP is stabilized by binding to heterogeneous nuclear ribonucleoproteins (hnRNPs).