Component of the Mediator complex, a coactivator involved in the regulated transcription of nearly all RNA polymerase II-dependent genes. Mediator functions as a bridge to convey information from gene-specific regulatory proteins to the basal RNA polymerase II transcription machinery. Mediator is recruited to promoters by direct interactions with regulatory proteins and serves as a scaffold for the assembly of a functional preinitiation complex with RNA polymerase II and the general transcription factors. Required for cholesterol-dependent gene regulation. Positively regulates the Nodal signaling pathway. Component of the Mediator complex, which is composed of MED1, MED4, MED6, MED7, MED8, MED9, MED10, MED11, MED12, MED13, MED13L, MED14, MED15, MED16, MED17, MED18, MED19, MED20, MED21, MED22, MED23, MED24, MED25, MED26, MED27, MED29, MED30, MED31, CCNC, CDK8 and CDC2L6/CDK11. The MED12, MED13, CCNC and CDK8 subunits form a distinct module termed the CDK8 module. Mediator containing the CDK8 module is less active than Mediator lacking this module in supporting transcriptional activation. Individual preparations of the Mediator complex lacking one or more distinct subunits have been variously termed ARC, CRSP, DRIP, PC2, SMCC and TRAP. Interacts with SMAD2, SMAD3, SREBF1 and SREBF2. Interacts with WWTR1. Interacts with TRIM11. Expressed in all tissues examined, including heart, brain, lung, spleen, thymus, pancreas, blood leukocyte and placenta. However, the level of expression varied, with highest expression in the placenta and peripheral blood and lowest in the pancreas and kidney. By 12-O-tetradecanoylphorbol-13-acetate (TPA). Ubiquitinated by TRIM11, leading to proteasomal degradation. The poly-Gln region from amino acids 235-262 of PCQAP is polymorphic. There are from 15 to 18 repeats in the Italian population. Belongs to the Mediator complex subunit 15 family. Several sequencing errors.