Single-Molecule Mechanistic Dissection of a Chromatin Remodeling Motor

Abstract

Chromatin remodeling complexes are molecular motors that catalyze diverse structural rearrangements of the genetic material in eukaryotic cells. One such motor, human ACF, is involved in many fundamental gene regulatory processes, including transcriptional activation and repression. ACF is composed of a motor subunit called SNF2h, which is shared with several other remodeling complexes, and a non-catalytic subunit called Acf1. Both SNF2h alone and the ACF complex slide nucleosomes, the basic unit of chromatin, along DNA. Despite the prevalence of ACF and related complexes in genomic regulation, much remains unclear about the mechanism of their sliding activity. SNF2h and Acf1 contain a number of “moving parts” that contribute to significant conformational rearrangements of the enzyme during the remodeling cycle, making them challenging to study by ensemble methods. SNF2h and ACF also dimerize on nucleosomes, further complicating their study in asynchronous populations. Single-molecule techniques are therefore particularly well-suited to investigating such a system. We use single-molecule FRET to observe SNF2h and ACF remodeling individual nucleosomes, to address the following questions: (1) How does the Acf1 accessory subunit modify the basic motor properties of SNF2h, especially in terms of processivity, directional commitment, and remodeling efficiency? (2) How do the “moving” parts of SNF2h combine to regulate the basic activity of the motor? (3) How is protomer coordination achieved across the nucleosome to avoid a tug of war, since enzyme monomers do not directly contact each other?