Estelle Hrabak, Ph.D.

Estelle Hrabak, Ph.D.

Associate Professor

Educational Background:

Ph.D., University of Wisconsin, 1992
B.S., Michigan State University, 1978

Courses Taught:

  • GEN 401 Professional Perspectives in Genetics
  • GEN 604 Principles of Genetics
  • GEN 606 Genetics Lab
  • BMCB 754/854 Laboratory in Biochemistry & Molecular Biology of Nucleic Acids

General Areas of Interest/Specialty:

Signal transduction; roles of protein phosphatases in plant growth and development; protein palmitoylation and subcellular protein targeting

General Research Interests:

The Hrabak lab studies intracellular signal transduction mechanisms involving phosphorylation or lipidation. 

Regulation of Proteins by Phosphorylation

Addition and removal of phosphate groups is a common mechanism for regulating protein function.  Phosphorylation is catalyzed by kinases and dephosphorylation is carried out by phosphatases.  The expression and activity of these enzymes must be tightly controlled to maintain normal cell function and to allow rapid and appropriate responses to stresses.  Both prokaryotic and eukaryotic genomes encode many protein kinases and protein phosphatases that target a wide variety of substrates.  (It is estimated that one-third of all proteins in eukaryotes are regulated by phosphorylation.)

Plants containing mutations are useful tools to dissect the functions of protein phosphatase 2A (PP2A) genes in plants.  PP2A is a heterotrimeric enzyme containing one A, one B, and one C subunit.  In most organisms, multiple genes encode each subunit and thus, many A-B-C combinations are possible.

Some pp2a mutant plants develop root growth defects in the presence of sodium ions, indicating that one function of PP2A is to maintain normal root growth under ionic stress conditions.

Arabidopsis pp2a mutant on medium without sodium

arabidopsis without sodium


Arabidopsis pp2a mutant on medium with sodium

arabidopsis with sodium

Current research areas:

  • determine the mechanisms underlying the root growth defect of pp2a mutants
  • identify other proteins involved in regulating root growth in a sodium-dependent manner
  • investigate the subunit composition of PP2A complexes in plants

Regulation of Proteins by Lipidation

Some proteins are modified by the addition of lipids.  One type of lipid modification termed S-acylation (also called palmitoylation) involves the addition of a fatty acid, often C16:0 palmitate, to a cysteine residue in the substrate protein.  S-acylation is catalyzed by palmitoyltransferases (PATs).  Palmitoyltransferases are integral membrane proteins that palmitoylate both cytosolic proteins and membrane proteins.  In Arabidopsis, more than 20 genes encode PATs, indicating that the enzymes may be specific for particular substrates or expressed in different tissues or at different developmental timepoints.  Palmitoylation defects in animals are implicated in some neurological disorders, cancers, and mental retardation.  In plants, the functions of the pat genes are just starting to be elucidated.

wild-type plant and pat mutant plant
A wild-type plant (left) and a
pat mutant plant

Current research areas:

  • identification of PAT substrates and PAT gene expression patterns
  • identification of the membrane location of PATs
  • characterization of plants with mutations in PAT genes

Potential Undergraduate Researchers:

Undergraduate students are invited to interview for research positions in the lab at the start of each semester.  Applicants should have a solid GPA and an interest in doing research in the areas of genetics, cell biology or biochemistry.  Contact Dr. Hrabak ( before the semester starts to learn about available positions.

Representative Publications:

Lu, S.X. and E.M. Hrabak. 2013. The myristoylated amino-terminus of an Arabidopsis calcium-dependent protein kinase mediates plasma membrane localization. Plant Mol. Biol. 82:267-287

Hegeman, A.D., M. Rodriguez, B.W. Han, Y. Uno, G.N. Phillips, E.M. Hrabak, J.C. Cushman, J.F. Harper, A.C. Harmon, and M.R. Sussman. 2006. A phyloproteomic characterization of in vitro autophosphorylation in calcium-dependent protein kinases.  Proteomics 6:3649-3664.

Dammann, C., A. Ichida, B. Hong, S. Romanowsky, E.M. Hrabak, A.C. Harmon, B.G. Pickard, and J.F. Harper. 2003. Subcellular targeting of nine calcium-dependent protein kinase isoforms from Arabidopsis. Plant Physiol. 132:1840-1848

Hrabak, E.M., C.W.M. Chan, M. Gribskov, J.F. Harper, J.H. Choi, N. Halford, J. Kudla, S. Luan, H.G. Nimmo, M.R. Sussman, M. Thomas, K. Walker-Simmons, J.-K. Zhu, and A.C. Harmon. 2003. The Arabidopsis CDPK-SnRK superfamily of protein kinases. Plant Physiol. 132:666-680

Lu, S.X. and E.M. Hrabak. 2002. An Arabidopsis calcium-dependent protein kinase is associated with the endoplasmic reticulum. Plant Physiol. 128:1008-1021

Hrabak, E.M. 2000. Calcium-dependent protein kinases and their relatives. Adv. Bot. Res. 32:185-223

Hrabak, E.M., L.J. Dickmann, J.S. Satterlee, and M.R. Sussman. 1996. Characterization of eight new members of the calmodulin-like domain protein kinase gene family of Arabidopsis thaliana. Plant Mol. Biol. 31:405-412


Rudman Hall, Room 105
Durham, NH 03824
(603) 862-0716