SWI/SNF

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File:SNF2-Like Family.jpg
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 (SWItch/Sucrose NonFermentable)[1][2] 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.[3] 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.

Family members

Below is a list of yeast SWI/SNF family members and human orthologs:[4]

yeast human function
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.[5] 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.[6] A recent study[7] has provided strong evidence against the twist diffusion mechanism and has further strengthened the loop-recapture model, as proposed in the figure below.

File:SWI-SNF Model.jpg
A study conducted by Zofall et al.[7] provided more evidence for the "bulge propagation" mechanism of nucleosome remodeling.

See also

References

  1. Neigeborn L, Carlson M (1984). "Genes affecting the regulation of SUC2 gene expression by glucose repression in Saccharomyces cerevisiae". Genetics. 108 (4): 845–58. PMID 6392017.
  2. Stern M, Jensen R, Herskowitz I (1984). "Five SWI genes are required for expression of the HO gene in yeast". J. Mol. Biol. 178 (4): 853–68. doi:10.1016/0022-2836(84)90315-2. PMID 6436497.
  3. Pazin MJ, Kadonaga JT (1997). "SWI2/SNF2 and related proteins: ATP-driven motors that disrupt protein-DNA interactions?". Cell. 88 (6): 737–40. doi:doi:10.1016/S0092-8674(00)81918-2 Check |doi= value (help). PMID 9118215.
  4. Collingwood TN, Urnov FD, Wolffe AP (1999). "Nuclear receptors: coactivators, corepressors and chromatin remodeling in the control of transcription". J. Mol. Endocrinol. 23 (3): 255–75. doi:10.1677/jme.0.0230255. PMID 10601972.
  5. van Holde K, Yager T (2003). "Models for chromatin remodeling: a critical comparison". Biochem. Cell Biol. 81 (3): 169–72. doi:10.1139/o03-038. PMID 12897850.
  6. Flaus A, Owen-Hughes T (2003). "Mechanisms for nucleosome mobilization". Biopolymers. 68 (4): 563–78. doi:10.1002/bip.10323. PMID 12666181.
  7. 7.0 7.1 Zofall M, Persinger J, Kassabov SR, Bartholomew B (2006). "Chromatin remodeling by ISW2 and SWI/SNF requires DNA translocation inside the nucleosome". Nat. Struct. Mol. Biol. 13 (4): 339–46. doi:10.1038/nsmb1071. PMID 16518397.



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