Robert M. Larkin
Assistant Professor
- Ph.D. 1996, University of Missouri
S206 Plant Biology Building
Michigan State University
East Lansing, MI 48824
Office: 517-432-4619
Lab: 517-432-4621
Fax: (517) 353-9168
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Robert M. Larkin Research Interests continued
Indeed, mutants with defects in this light-plastid signaling network regulate chloroplast biogenesis and function less effectively than wild type. For example, the cotyledons of mutants with defects in plastid-to-nucleus signaling are chlorophyll deficient and become more chlorophyll deficient than wild type when light intensities are increased (Figure 2).
Figure 2. Sensitivity of plastid-to-nucleus signaling mutants to light intensity. Wild type or the indicated mutants performed chloroplast biogenesis in 100, 500, 1000, and 1500 mmol m-2 s-1 white light. Representative cotyledons are shown. Representative chlorophyll deficient cotyledons that form at a low frequency in 100 mmol m-2 s-1 white light are also shown for gun1 and gun1 cry1.
Our second project is focused on chlorophyll biosynthesis. Because accumulation of the chlorophyll precursor Mg-protoporphyrin IX within plastids is a plastid signal that regulates PhANG expression, our work on chlorophyll metabolism is related to our studies on PhANG expression. We are studying a novel regulator of chlorophyll biosynthesis named GUN4. GUN4 participates in a plastid-to-nucleus signaling pathway that is triggered by Mg-protoporphyrin IX and exhibits no sequence similarity to proteins with known functions. In addition to affecting plastid-to-nucleus signaling pathways, GUN4 is a major regulator of chlorophyll biosynthesis. GUN4 regulates chlorophyll metabolism by stimulating Mg-chelatase, the enzyme that commits protoporphyrin IX to chlorophyll biosynthesis. GUN4 helps Mg-chelatase utilize its protoporphyrin IX substrate more effectively and facilitates the release of Mg-protoporphyrin IX from Mg-chelatase using a mechanism that involves binding to GUN5 and to the substrate and product of Mg-chelatase (i.e., protoporphyrin IX and Mg-protoporphyrin IX) (Figure 3).
Figure 3. The novel GUN4 Core domain fold is a porphyrin-binding domain that resembles an all helical hand in a cupped posture. Our studies indicate that GUN4 stimulates Mg-chelatase and binds porphyrins before and after metal insertion by Mg-chelatase. The figure shows the GUN4 Core domain from a 1.78-Å crystal structure of a GUN4 relative from Synechocystis and a porphyrin modeled into the palm region of the GUN4 Core domain. The porphyrin-binding site shown in the model is supported by our structure-function studies. (Figure drawn by Mark Verdecia.)
Recent Publications
Ruckle ME, Demarco SM, Larkin RM (2007) Plastid signals remodel light signaling networks and are essential for efficient chloroplast biogenesis in Arabidopsis. Plant Cell 19: 3944-3960. Link to pdf
Verdecia MA, Larkin RM, Ferrer JL, Riek R, Chory J, Noel JP (2005) Structure of the Mg-Chelatase Cofactor GUN4 Reveals a Novel Hand-Shaped Fold for Porphyrin Binding. PLoS Biol. 3(5):e151 DOI: 10.1371/journal.pbio.0030151. Link to pdf
Larkin, R.M., Alonso, J.M., Ecker, J.R. and Chory, J. (2003) GUN4, a regulator of chlorophyll synthesis and intracellular signaling. Science 299, 902-906. Link to pdf
Mochizuki N., Brusslan J.A., Larkin R., Nagatani, A. and Chory, J. (2001) Arabidopsis genomes uncoupled 5 (GUN5) mutant reveals the involvement of Mg-chelatase H subunit in plastid-to-nucleus signal transduction. Proc. Natl. Acad. Sci. USA 98, 2053-2058. Link to pdf