SWI/SNF

From Wikipedia, the free encyclopedia

Diagram indicating location of the SNF2 Subfamily of proteins and their relative similarity with other proteins organized by their conservation of NTP-binding motifs.

SWI/SNF is a nucleosome remodeling complex composed of several proteins - products of the SWI and SNF genes (SWI1, SWI2/SNF2, SWI3, SNF5, SNF6) as well as several other polypeptides.^[1] It possesses a DNA-stimulated ATPase activity and can destabilize histone-DNA interactions in reconstituted nucleosomes in an ATP-dependent manner, though the exact nature of this structural change is unknown.

[edit] Mechanism of Action

Two mechanisms for nucleosome remodeling by SWI/SNF have been proposed.^[2] The first model contends that a unidirectional diffusion of a twist defect within the nucleosomal DNA results in a corkscrew-like propagation of DNA over the octamer surface that initiates at the DNA entry site of the nucleosome. The other is known as the "bulge" or "loop-recapture" mechanism and it involves the dissociation of DNA at the edge of the nucleosome with reassociation of DNA inside the nucleosome, forming a DNA bulge on the octamer surface. The DNA loop would then propagate across the surface of the histone octamer in a wave-like manner, resulting in the repositioning of DNA without changes in the total number of histone-DNA contacts.^[3] A recent study^[4] has provided strong evidence against the twist diffusion mechanism and has further strengthened the loop-recapture model, as proposed in the figure below.