Characterization of elongin C functional domains required for interaction with elongin B and activation of elongin A.

The Elongin (SIII) complex stimulates the rate of elongation by RNA polymerase II by suppressing transient pausing by polymerase at many sites along DNA templates. The Elongin (SIII) complex is composed of a transcriptionally active A subunit, a chaperone-like B subunit, which promotes assembly and enhances stability of the Elongin (SIII) complex, and a regulatory C subunit, which (i) functions as a potent activator of Elongin A transcriptional activity, (ii) interacts specifically with Elongin B to form an isolable Elongin BC complex, and (iii) is bound and negatively regulated in vitro by the product of the von Hippel-Lindau tumor suppressor gene. As part of our effort to understand how Elongin C regulates the activity of the Elongin (SIII) complex, we are characterizing Elongin C functional domains. In this report, we identify Elongin C mutants that fall into multiple functional classes based on their abilities to bind Elongin B and to bind and activate Elongin A under our assay conditions. Characterization of these mutants suggests that Elongin C is composed of multiple overlapping regions that mediate functional interactions with Elongin A and B.