SWI/SNF

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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.

A study conducted by Zofall et al. provided more evidence for the "bulge propagation" mechanism of nucleosome remodeling.

[edit] See also

• Chromatin Structure Remodeling (RSC) Complex

[edit] References

1. ^ Pazin MJ, Kadonaga JT, "SWI2/SNF2 and Related Proteins: ATP-Driven Motors That Disrupt Protein-DNA Interactions?", Cell, (1997) Mar 21; 88: 737-740.
2. ^ van Holde K, Yager T, "Models for chromatin remodeling: a critical comparison", Biochemical Cell Biology. (2003); 81: 169-172.
3. ^ Flaus A, Owen-Hughes T., "Mechanisms for nucleosome mobilization", Biopolymers. (2003); 68: 563-578.
4. ^ Zofall M, Persinger J, Kassabov SR, Bartholomew B, "Chromatin remodeling by ISW2 and SWI/SNF requires DNA translocation inside the nucleosome", Nature Structural & Molecular Biology. (2006) Published Online March 5.

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