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Graham H Thomas

Graham H Thomas

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Associate Professor of Biology and Biochemistry and Molecular Biology

617 Mueller Lab
University Park, PA 16802
Phone: (814)863-0716
Lab Address: 607 Mueller Lab
Lab Phone: (814) 863-0718
Email:

Education

  1. Ph.D., University of Edinburgh, United Kingdom, 1984
  2. B.S., University of Sheffield, United Kingdom, 1980

Postdoc Training

  1. Harvard University, 1988-1993
  2. Washington University in St. Louis, 1984-1988

Research Interests

Role of the cortical actin cytoskeleton in cell polarity and morphogenesis.

Research in my lab asks fundamental questions about the roles of the cytoskeleton at the cell membrane in epithelial cells, including issues of cell polarity and adhesion, cell signaling, and morphogenesis. Drosophila is our model system because of the multidisciplinary combination of tools available and because of its well-characterized development. We use both molecular and cellular techniques as well as classical and transgenic genetic approaches.

The spectrin-based membrane skeleton is a ubiquitous structure that is conserved in diverse organisms. Spectrins are long heterotetramers of two a and two b chains, which crosslink F-actin and contain numerous protein binding sites along their length. Typically, different spectrins are polarized to distinct parts of the plasma membrane. Drosophila provides a simple model system for examining this molecular scaffold, since the fly has only one a and two b genes: the type of spectrin thus depends on which b chain is used. Our goal is to understand how differentiation in the membrane skeleton is used to polarize cells in a developmental context.

We are currently focusing on one of these b-spectrin isoforms, b[Heavy]-spectrin (bH), which is associated with the zonula adherens, apical microvillar fields, and morphogenetic movements driven by cytoplasmic myosin II. The distribution of bH during early embryogenesis suggests a role in early events that results in cell polarization, and mutations in the locus encoding this protein cause a number of defects in tissues of epithelial origin, including failure of at least one polarized signaling pathway that leads to a specific cell fate defect.

Our most recent phenotypic analysis of the karst mutation and careful immunofluorescent studies on the behavior of both b-spectrins during primary epithelium formation have revealed two significant results. First, apical and basolateral spectrin behave in quite distinct ways that suggest different, rather than truly analogous, roles in their respective domains, as many have assumed. Second, apical spectrin is necessary for normal apical contraction (a classic cell shape change that is required for generating form in epithelia) and for maintaining the integrity of the zonula adherens. Suprisingly, bH function is not closely associated with the apicobasal polarization pathway; karst mutants can generate and maintain epithelia with bona fide apicobasal polarity. Thus, we are beginning to redefine and clarify some of the long-established notions concerning the roles of the spectrin-based membrane skeleton in epithelia.

We also maintain an interest in the evolutionary origins of the membrane skeleton through collaboration with Dr. Andrew Clark (Penn State) and Dr. Spencer Muse (North Carolina State University). This collaboration has generated a comprehensive model for the evolution of the a-actinin/spectrin/dystrophin superfamily of proteins. We have found evidence that the ancestral gene structures of this superfamily were unstable during an early phase in their evolution and that this phase was dominated by concerted evolution. This has been followed by long-term stability in gene organization and a lack of sequence exchange between them. This model has general applicability for other proteins with repetitive structures. We also are attempting to identify novel functionality in known spectrin proteins through the analysis of regional differences in evolutionary rate within these proteins. Eventually, such analyses may provide new directions for our molecular analyses as well as some insight into the origins of morphogenetic processes involving these proteins.

My laboratory provides training in a variety of techniques that have wide applicability to other experimental systems. Furthermore, our multidisciplinary approach means that a typical experiment might include several of these. Experiments currently in progress use standard molecular biological techniques (such as PCR, cloning, sequencing and bacterial protein expression), the generation of a transgenic flies expressing mutant proteins, immunofluorescent microscopy with digital image acquisition, and the analysis of genetic interactions.

Selected Publications

  • M├ędina, E., Williams, J.A., Klipfell, E.A., Zarnescu, D.C., Thomas, G.H., Le Bivic, A. 2002. Crumbs interacts with a Moesin-bHeavy-Spectrin membrane skeleton in Drosophila. J. Cell Biol. 158(5) in press. (9/2/02).
  • Thomas, G.H. and Discher, D.E. 2002. Conformational compliance of spectrins in membrane deformation, morphogenesis and signaling. in 'Elastic Proteins', P.R. Shewry, A. Tatham and A.J. Bailey, eds. Cambridge University Press, Cambridge, U.K. in press<
  • Thomas, G.H. 2001. Spectrin: the ghost in the machine. BioEssays 23;152-160.
  • Thomas, G.H. and Williams, J.A. J. 1999. Dynamic rearrangement of the spectrin membrane skeleton during the generation of epithelial polarity in Drosophila. .
  • Zarnescu, D.C. and Thomas, G.H. 1999. Apical spectrin is essential for epithelial morphogenesis but not apico-basal polarity in Drosophila. J. Cell Biol. 146;1075-1086.
  • Thomas, G.H., Zarnescu, D.C, Juedes, A.E., Bales, M.A., Londergan, A., Korte, C.C. and Kiehart, D.P. 1998. Drosophila bHeavy-spectrin is essential for development and contributes to specific cell fates in the eye. Development 125; 2125-2134.
  • Thomas, G.H., Newbern, C., Korte, C.C., Bales, M.A., Kiehart, D.P.and Clark, A.G. 1997. Intragenic duplication and divergence in the spectrin superfamily of proteins. Mol. Biol. Evol. 14; 1285-1295.
  • Thomas, G.H. and Kiehart D.P. 1994. bHeavy spectrin has a restricted tissue and subcellular distribution during Drosophila development. Development. 120;2039-2050.