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Sarah Bergeron

Mentor: Peter Rubenstein, Ph.D.
Lab Room: 4-411 BSB
Lab Phone: 319-335-7912

Biochemical Effects of Deafness-Causing gamma-Actin Mutations on Actin functions

Eight different point mutations in mammalian γ-nonmuscle actin have been shown to cause autosomal dominant deafness, and a number of these mutations are in regions of actin that may affect binding sites for proteins that regulate filament formation and stability. Hair cell actin is composed of β and γ nonmuscle actin. Hair cells are one of the few cell types in which the γ isoform predominates over the β form (2:1 ratio of γ to β). The molecular mechanism leading to this actin-dependent deafness phenotype is unknown. Although each mutant causes the similar deafness phenotype in humans, once cloned into yeast they cause allele-specific phenotypes. Five of the six mutants demonstrate near normal polymerization kinetics, leading us to hypothesize that the deafness phenotype may be largely due to altered actin/actin-binding-protein interaction(s). Utilizing the Baculovirus system we have begun an initial characterization of human γ- and β-nonmuscle actin. Results show that γ-nonmuscle actin has a similar thermal stability, critical concentration, and filament morphology to that of yeast actin. However, its polymerization and nucleotide exchange rates are both slower than those of yeast actin. Surprisingly, β-nonmuscle actin’s polymerization rate and phosphate release rate is two times faster than γ-nonmuscle actin and is closer to that of yeast actin. These results are unexpected because γ- and β-nonmuscle actin differ by only 4 amino acids, three of which are in the amino-terminus. Although surprising, the polymerization rates do correspond to the localization of the isoforms within motile cells; β-nonmuscle actin in the dynamic leading edge of the cell and γ-nonmuscle actin in the more stationary cell center and stress fibers. While these isoforms are separate within the cell, in-vitro they are able to copolymerize creating a mixed filament with β-actin predominating over γ-actin. Once the in-vitro biochemical properties have been established for γ- and β- nonmuscle actin, all deafness causing γ-nonmuscle actin mutants that previously exhibited abnormal protein interactions in the yeast background will be tested in the human background.

Malloy LE, Wen KK, Pierick AR, Wedemeyer EW, Bergeron SE, Vanderpool ND, McKane M, Rubenstein PA, Bartlett HL. Thoracic Aortic Aneurysm (TAAD)-causing Mutation in Actin Affects Formin Regulation of Polymerization. J Biol Chem. 2012 Aug 17;287(34):28398-408. Epub 2012 Jun 29. PubMed PMID: 22753406; PubMed Central PMCID: PMC3436569.

Bergeron SE, Wedemeyer EW, Lee R, Wen KK, McKane M, Pierick AR, Berger AP, Rubenstein PA, Bartlett HL. Allele-specific effects of thoracic aortic aneurysm and dissection alpha-smooth muscle actin mutations on actin function. J Biol Chem. 2011 Apr 1;286(13):11356-69. Epub 2011 Feb 2. PubMed PMID: 21288906; PubMed Central PMCID: PMC3064192.

Bergeron SE, Zhu M, Thiem SM, Friderici KH, Rubenstein PA. Ion-dependent polymerization differences between mammalian beta- and gamma-nonmuscle actin isoforms. J Biol Chem. 2010 May 21;285(21):16087-95. Epub 2010 Mar 22. PubMed PMID: 20308063; PubMed Central PMCID: PMC2871477.

Shvetsov A, Galkin VE, Orlova A, Phillips M, Bergeron SE, Rubenstein PA, Egelman EH, Reisler E. Actin hydrophobic loop 262-274 and filament nucleation and elongation. J Mol Biol. 2008 Jan 18;375(3):793-801. Epub 2007 Nov 4. PubMed PMID:18037437.

Honors and Awards

  • 2007 MCB Retreat Travel Award for Poster Presentation