Anita Hopper

Focus

Intracellular trafficking of RNA and proteins; Nucleus organization; RNA processing

Research interests

Our research focuses on the intersection of genetics and eukaryotic cell biology. Using yeast, S. cerevisiae, as a model system we employ a multidiscipline approach to study the mechanisms of subcellular distribution of RNAs and proteins. Our current focus is on the nucleus.

One current project concerns the inner nuclear membrane. The nucleus has a complex structure consisting of an outer (ONM) and inner (INM) membrane, connected by aqueous channels (nucleopores), as well as areas of specialization such as the nucleolus and cajal bodies. In metazoans the nucleus disassembles each cell cycle and must then be reassembled with precision. How the nucleus assembles and how its structure is maintained is poorly understood. We study a yeast protein that locates at the INM (Trm1) and employ genomic-wide methods to identify motifs directing this protein to the INM and the proteins/lipids that tether or regulate its INM location. We discovered three unanticipated categories of gene products that play important roles in INM organization: (1) N-terminal protein acetylation by Nat C; (2) Ice2, an integral ER protein that may serve as a regulator of INM tethers; (3) components of the spindle pole body, the yeast equivalent of the centrosome. We are optimistic that our studies of the yeast INM will also unravel complexities of metazoan nuclear structure and biogenesis.

A second project concerns intracellular trafficking of tRNAs. Nearly all RNAs that function in protein synthesis are generated in the nucleus, but function in the cytoplasm. Conversely, many proteins generated in the cytoplasm, function in the nucleus. We are using genetic, genome-wide, biochemical, and cell biological approaches to dissect this fascinating, complex process. We have learned that there are parallel nuclear export pathways, that export is quality controlled by components of the translation machinery that previously were thought to reside exclusively in the cytoplasm, and that nucleus/cytoplasm exchange and cellular metabolism are intricately connected. Most recently, we showed that tRNAs move retrograde from the cytoplasm to the nucleus under particular stress conditions - a surprising and anti-dogmatic discovery. Our current studies indicate that retrograde tRNA movement also occurs in metazoans. We are testing the hypothesis that tRNA retrograde movement serves to regulate protein synthesis under conditions of nutrient stress by separating tRNA from the cytosplasmic translation machinery.

 

Publications

• Shaheen, H. H., R. L. Horetsky, S.R. Kinball, A. Murthi, L.S. Jefferson and A. K. Hopper. Cytoplasmic tRNA is imported into nuclei of rat hepatoma cells in response to nutrient deprivation. Proc. Natl. Acad. Sci. 104:8845-8850 (2007).
• Hurto, R.L., A. Tong, C. Boone, and A.K. Hopper. Inorganic phosphate deprivation causes tRNA nuclear accumulation via retrograde transport in S. cerevisiae. Genetics 176: 841-852 (2007) highlighted article and issue cover image.
• Whitney, M.L., R.L. Hurto, H.H. Shaheen, and A.K. Hopper. Rapid and reversible nuclear accumulation of cytoplasmic tRNA in response to nutrient availability. Mol.Biol. Cell 18: 2678-2686 (2007).
• Hopper, A.K. Cellular Dynamics of small RNAs. Critical Rev. Biochem. Mol. Biol. 41:3-19 (2006).
• Butterfield-Gerson, K. L., L. Z. Scheifele, E.P. Ryan, A. K. Hopper, and L. J. Parent. Importin-? family members mediate alpharetrovirus Gag nuclear entry via interactions with matrix and nucleocapsid. J. Virology 80:1798-1806 (2006).
• Murthi, A. & A.K. Hopper. Genome-wide screen for inner nuclear membrane protein targeting in Saccharomyces cerevisiae: reveals roles for N-acetylation and an integral protein. Genetics 170:1553-1560
• Shaheen, H.H. and A.K. Hopper. Retrograde movement of tRNAs from the cytosplasm to the nucleus in S. cerevisiae, Proc. Natl. Acad. Sci., 102:11290-11295 (2005).
• Go, W., R.L. Hurto, A.K. Hopper, E.J. Grayhack, and E.M. Phizicky. Depeletion of yeast tRNAHis guanylyltransferase Thg1p leads to uncharged tRNAHis with additional m5C. Mol. Cell Biol. 25:8191-8201 (2005).
• Kwapisz, M, P. Cholbinski, A.K. Hopper, J.-P. Rousset, and T. Zoladek. Rsp5p dependent ubiquitination modulates translation accuracy in yeast Saccharomyces cerevisae. RNA 11:1710-1718 (2005).
• Hopper, A.K. & E.M. Phizicky. tRNA transfers to the limelight. Genes Dev. 17: 162-180 (2003).
• Feng, W. & A.K. Hopper. A Los1p-independent pathway for nuclear export on intronless tRNAs in S. cerevisiae. Proc. Nat. Acad. Sci. USA 99: 5412-5417 (2002).