SWI/SNF (SWItch/Sucrose NonFermentable) is a yeast nucleosome remodeling complex composed of several proteins - products of the SWI and SNF genes (SWI1, SWI2/SNF2, SWI3, SWI5, SWI6) as well as several other polypeptides. 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.
|SWI1||ARID1A, ARID1B||contains LXXLL nuclear receptor binding motifs|
|SWI2/SNF2||SMARCA4||ATP dependent chromatin remodeling|
|SWI3||SMARCC1, SMARCC2||similar sequence, function unknown|
|SWP73||SMARCD1, SMARCD2, SMARCD3||similar sequence, function unknown|
|SWP61||ACTL6A, ACTL6b||actin-like protein|
Mechanism of Action
Two mechanisms for nucleosome remodeling by SWI/SNF have been proposed. 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. A recent study has provided strong evidence against the twist diffusion mechanism and has further strengthened the loop-recapture model, as proposed in the figure below.
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