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| FACULTY BIOGRAPHY |
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| Tulle Hazelrigg |
| Research Scientist |
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| My lab uses the genetic model organism Drosophila to address basic questions about the propagation and differentiation of germ cells. Germ cells protect, rearrange and pass on a species’ genome to the next generation, and also undergo a complex differentiation process to become highly specialized cell types, the gametes. In Drosophila oogenesis, germ stem cells make binary cell fate choices each time they divide, followed by a complex differentiation program that produces the oocyte. We are interested in how genes are regulated during these events, and how determinants are spatially ordered and stored during the differentiation of the oocyte.
Subcellular RNA localization is a fundamental mechanism used by cells to create cell polarity. In the case of the Drosophila oocyte, mRNA localization to either the anterior or posterior poles of the oocyte is a prerequisite for anterior-posterior patterning of the embryo. For example, bicoid (bcd) mRNA is localized at the oocyte anterior pole where its protein product, a transcription factor, regulates head and thorax development in the embryo. Our studies on bcd RNA localization have focused on the roles of trans-acting factors, including the products of the exu and yps genes, and the microtubule cytoskeleton. During our studies we produced the first fusion protein with Green Fluorescent Protein (GFP), GFP-Exu, and discovered that Exu protein forms large cytoplasmic particles that are transported along microtubules in the germ cells. In collaboration with Ron Vale and Jim Wilhelm, at UCSF, we purified an Exu RNP complex, using the GFP tag on Exu as a biochemical handle. These studies led to the identification of a ribonucleoprotein complex (RNP) that contains Exu, 7 other proteins, and mRNAs. Our discovery of osk mRNA in this complex, an RNA localized at the posterior pole of the oocyte, was a surprise to us, and supported the idea that there are links between the pathways that target mRNAs to the different poles of the egg.
A second level of regulation is often imposed on localized mRNAs to ensure that these RNAs are only translated at their sites of localization, thus tightening the restriction of their protein products to specific cellular domains. One of the proteins we identified in the Exu complex, Ypsilon Schachtel (encoded by the yps gene), is a Y-Box protein. Since other Y-box proteins repress translation, we hypothesized that Yps may serve a similar function during oogenesis. Our genetic analysis of yps revealed that it does indeed regulate the translation of osk mRNA at the posterior pole of the oocyte. Thus the Exu particles have dual roles in regulating both RNA localization and translation.
In our studies on the role of microtubules in RNA localization, we found that the mini-spindles (msps) gene , which encodes a microtubule-associated protein (MAP), is required for bcd RNA localization. We showed that msps is required for a specialized population of microtubules that appear to originate in the oocyte and extend through the ring canals at mid-oogenesis. This work demonstrated for the first time an essential role for a classic MAP in subcellular RNA localization. Together our results support a model in which bcd RNA is transported along specialized microtubules into the oocyte, where microtubules further serve to anchor the RNA at the anterior pole.
Recently we have begun to work on the role of histone methylation in oogenesis. Histone methyltransferases contain a catalytic domain known as the SET domain. In a mutagenesis screen for female-sterile mutations, we identified a gene that encodes a SET domain protein, and named this gene eggless (egg). We showed that egg is required for tri-methylation of histone H3 at its K9 residue (H3-K9), and that this activity is especially strong in the germarium, the ovary structure where germ stem cells reside, and where egg chambers are formed. In egg mutant ovaries, early germ cell differentiation is defective, and genes that are normally expressed in germ stem cells and then down-regulated as germ cells begin to differentiate, are de-repressed. We are testing the hypothesis that the Egg protein normally establishes repressive chromatin domains at the promoters of specific genes, thereby establishing a necessary temporal pattern of gene expression in germ stem cells and their daughters. Current experiments in the lab are focused on identifying the direct gene targets of Egg, and the effector proteins that bind the Egg methylation signal and interpret its consequences for gene expression.
MedLine Listing of Dr. Hazelrigg's Publications
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Representative Recent Publications
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- Clough, E., Moon, W., Wang, S., Smith, K., and Hazelrigg, T. (2007) Histone methylation is required for oogenesis in Drosophila. Development. Jan 134(1): 157-65.
- Hazelrigg, T., and Mansfield, J H. (2006) Green Fluorescent Protein Applications in Drosophila. Methods Biochem Anal 47: 227-57.
- Moon, W., and Hazelrigg, T. (2004) The Drosophila microtubule-associated protein Mini Spindles is required for cytoplasmic microtubules in oogenesis. Curr. Biol 14: 1957-1961.
- Mansfield, J., Wilhelm, J.E., and Hazelrigg, T. (2002) Ypsilon Schactel, a Drosophila Y-box protein, acts antagonistically to Orb in the oskar mRNA localization and translation pathway Development 129: 197-209.
- Brent, A., MacQueen, A., and T. Hazelrigg (2000) The Drosophila wispy gene is required for RNA localization and other microtubule-based events of meiosis and early embryogenesis Genetics 154: 1649-1662.
- Wilhelm, J., Mansfield, J., Hom-Booher, N., Wang, S., Turck, C.W., Hazelrigg, T., and Vale, R. (2000) Isolation of a Ribonucleoprotein complex involved in mRNA localization in Drosophila oocytes J. Cell Biol. 148: 427-440.
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