File:PDB 1pd0 EBI.jpg|
crystal structure of the copii coat subunit, sec24, complexed with a peptide from the snare protein sed5 (yeast syntaxin-5)
|Symbols||; ADF; AGEL|
|RNA expression pattern|
|File:PBB GE GSN 200696 s at tn.png|
|File:PBB GE GSN 214040 s at tn.png|
Gelsolin is an actin-binding protein that is a key regulator of actin filament assembly and disassembly. Gelsolin is one of the most potent members of the actin-severing gelsolin/villin superfamily, as it severs with nearly 100% efficiency.  Gelsolin is located intracellularly (in cytosol and mitochondria) and extracellularly (in blood plasma).
Gelsolin is an 82-kD protein with six homologous subdomains, referred to as S1-S6. Each subdomain is composed of a five-stranded β-sheet, flanked by two α-helices, one positioned perpendicular with respect to the strands and one positioned parallel. The N-terminal (S1-S3) forms an extended β-sheet, as does the C-terminal (S4-S6).
Among the lipid-binding actin regulatory proteins, gelsolin (along with cofilin) is one of the few that exhibit preferential binding toward polyphosphoinositide (PPIs). The binding sequences in gelsolin closely resemble the motifs in the other PPI-binding proteins.
Gelsolin's activity is stimulated by calcium ions (Ca2+). Although the protein retains its overall structural integrity in both activated and deactivated states, the S6 helical tail moves like a latch depending on the concentration of calcium ions. The C-terminal end detects the calcium concentration within the cell. When there is no Ca2+ present, the tail of S6 shields the actin-binding sites on one of S2's helices. When a calcium ion attaches to the S6 tail, however, it straightens, exposing the S2 actin-binding sites. The N-terminal is directly involved in the severing of actin. S2 and S3 bind to the actin before the binding of S1 severs actin-actin bonds and caps the barbed end.
Gelsolin can be inhibited by a local rise in the concentration of phosphatidylinositol (4,5)-bisphosphate (PIP2), a PPI. This is a two step process. Firstly, (PIP2) binds to S2 and S3, inhibiting gelsolin from actin side binding. Then, (PIP2) binds to gelsolin’s S1, preventing gelsolin from severing actin, although (PIP2) does not bind directly to gelsolin's actin-binding site.
Gelsolin also inhibits apoptosis by stabilizing the mitochondria. Prior to cell death, mitochondria normally lose membrane potential and become more permeable. Gelsolin can impede the release of cytochrome C, obstructing the signal amplification that would have led to apoptosis.
Research in mice suggests that gelsolin, like other actin-severing proteins, is not expressed to a significant degree until after the early embryonic stage—approximately 2 weeks in murine embryos. In adult specimens, however, gelsolin is particularly important in motile cells, such as blood platelets. Mice with null gelsolin-coding genes undergo normal embryonic development, but the deformation of their blood platelets reduced their motility, resulting in a slower response to wound healing.
An insufficiency of gelsolin in mice has also been shown to cause increased permeability of the vascular pulmonary barrier, suggesting that gelsolin is important in the response to lung injury.
Sequence comparisons indicate an evolutionary relationship between gelsolin, villin, fragmin and severin. Six large repeating segments occur in gelsolin and villin, and 3 similar segments in severin and fragmin. While the multiple repeats have yet to be related to any known function of the actin-severing proteins, the superfamily appears to have evolved from an ancestral sequence of 120 to 130 amino acid residues. 
Gelsolin is a cytoplasmic, calcium-regulated, actin-modulating protein that binds to the barbed ends of actin filaments, preventing monomer exchange (end-blocking or capping). It can promote nucleation (the assembly of monomers into filaments), as well as sever existing filaments. In addition, this protein binds with high affinity to fibronectin. Plasma gelsolin and cytoplasmic gelsolin are derived from a single gene by alternate initiation sites and differential splicing.
Gelsolin has been shown to interact with:
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- Varon C, Tatin F, Moreau V, Van Obberghen-Schilling E, Fernandez-Sauze S, Reuzeau E et al. (May 2006). "Transforming growth factor beta induces rosettes of podosomes in primary aortic endothelial cells". Mol. Cell. Biol. 26 (9): 3582–94. PMC 1447430. PMID 16611998. doi:10.1128/MCB.26.9.3582-3594.2006.
- Kusano H, Shimizu S, Koya RC, Fujita H, Kamada S, Kuzumaki N et al. (October 2000). "Human gelsolin prevents apoptosis by inhibiting apoptotic mitochondrial changes via closing VDAC". Oncogene 19 (42): 4807–14. PMID 11039896. doi:10.1038/sj.onc.1203868.
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- Way M, Weeds A (October 1988). "Nucleotide sequence of pig plasma gelsolin. Comparison of protein sequence with human gelsolin and other actin-severing proteins shows strong homologies and evidence for large internal repeats". J. Mol. Biol. 203 (4): 1127–33. PMID 2850369. doi:10.1016/0022-2836(88)90132-5.
- Weeds AG, Gooch J, Pope B, Harris HE (November 1986). "Preparation and characterization of pig plasma and platelet gelsolins". Eur. J. Biochem. 161 (1): 69–76. PMID 3023087. doi:10.1111/j.1432-1033.1986.tb10125.x.
- Chauhan VP, Ray I, Chauhan A, Wisniewski HM (May 1999). "Binding of gelsolin, a secretory protein, to amyloid beta-protein". Biochem. Biophys. Res. Commun. 258 (2): 241–6. PMID 10329371. doi:10.1006/bbrc.1999.0623.
- Nishimura K, Ting HJ, Harada Y, Tokizane T, Nonomura N, Kang HY et al. (August 2003). "Modulation of androgen receptor transactivation by gelsolin: a newly identified androgen receptor coregulator". Cancer Res. 63 (16): 4888–94. PMID 12941811.
- Wang Q, Xie Y, Du QS, Wu XJ, Feng X, Mei L et al. (February 2003). "Regulation of the formation of osteoclastic actin rings by proline-rich tyrosine kinase 2 interacting with gelsolin". J. Cell Biol. 160 (4): 565–75. PMC 2173747. PMID 12578912. doi:10.1083/jcb.200207036.