Our current efforts are divided between two areas of study: (1) the molecular mechanism(s) of transcriptional repression mediated by the hairy basic-Helix-Loop-Helix transcriptional regulator, and (2) actin cytoskeletal dynamics mediated by the Rho1 small GTPase.

hairy is a pair rule segmentation gene that is essential for the establishment of reiterated pattern in the embryo. Hairy belongs to a subclass of basic helix-loop-helix (bHLH) transcription factors that function as dedicated repressors. Despite the genetic and molecular characterization to date, it has proved difficult to define the precise molecular mechanisms of Hairy action during segmentation. Our earlier work highlighted the requirement of multiple Hairy domains for its proper function, suggesting that Hairy is likely to be involved in multiple protein-protein interactions. We have identified several new Hairy-interacting proteins: dCtBP, a transcriptional cofactor implicated in both transcriptional activation and repression; dSir2, a NAD+-dependent histone deacetylase; and dNC2, a negative regulator of basal transcription. We are currently characterizing Hairy and its interacting proteins to help distinguish amongst the possible regulatory mechanisms used by Hairy to mediate transcriptional repression.

Rho GTPases play a central role in diverse biological processes such as actin cytoskeleton organization, microtubule dynamics, gene transcription, oncogenic transformation, adhesion and epithelial wound repair. We have characterized a loss-of-function mutation in the Drosophila RhoA homologue, Rho1. Loss of Rho1 function results in both maternal and embryonic phenotypes that are distinct from phenotypes described previously for the dominant negative and constitutively active Rho1 mutations. Embryos homozygous for the Rho1 mutation exhibit severe defects in head involution and imperfect dorsal closure. Two phenotypes are associated with reduction of maternal Rho1 activity: the actin cytoskeleton is disrupted in egg chambers, and the resulting embryos display patterning defects. Recent studies suggest that Rho may act through multiple separable pathways to carry out its various functions. While a handful of Rho targets have been identified in different systems to date, the molecular pathways in which each is involved largely remain to be elucidated. We are currently investigating the molecular mechanisms associated with two specific dRho1 regulatory pathways mediated through its interaction with ?lpha- and p120- catenin, proteins involved in adherens junction formation, or mediated through its interaction with Cappuccino, a formin homology protein important for actin and microtubule cytoskeleton integrity.

Recent Publications:

Rosenberg, M.I. and Parkhurst, S.M. (2002). Drosophila Sir2 is required for heterochromatic silencing and by the euchromatic Hairy/E(spl) family of bHLH repressors in segmentation and sex determination. Cell 109, 447-458.

Magie, C.R., Pinto-Santini, D. and Parkhurst, S.M. (2002). Rho1 interacts with p120ctn and a-catenin, and regulates cadherin-based adherens junction formation during Drosophila development. Development 129, 3771-3782