SWI/SNF
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SWI/SNF is a nucleosome remodeling complex composed of several of the SWI and SNF genes (SWI1, SWI2/SNF2, SWI3, SNF5, SNF6) as well as several other polypeptides. Pazin MJ, Kadonaga JT, [and Related Proteins: ATP-Driven Motors That Disrupt Protein-DNA Interactions?"], Cell, (1997) Mar 21; 88: 737-740. 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.
Mechanism of Action
Two mechanisms for nucleosome remodeling by SWI/SNF have been proposed. van Holde K, Yager T, [for chromatin remodeling: a critical comparison"], Biochemical Cell Biology. (2003); 81: 169-172. 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. Flaus A, Owen-Hughes T., [for nucleosome mobilization"], Biopolymers. (2003); 68: 563-578. A recent study Zofall M, Persinger J, Kassabov SR, Bartholomew B, [remodeling by ISW2 and SWI/SNF requires DNA translocation inside the nucleosome"], Nature Structural & Molecular Biology. (2006) Published Online March 5. has provided strong evidence against the twist diffusion mechanism and has further strengthened the loop-recapture model, as proposed in the figure below.
See also
References
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