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Molecular Plant Advance Access originally published online on December 4, 2007
Molecular Plant 2008 1(1):178-194; doi:10.1093/mp/ssm017
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© The Author 2007. Published by Oxford University Press on behalf of CSPP and IPPE, SIBS, CAS.

In Vivo Phosphorylation Site Mapping and Functional Characterization of Arabidopsis Phototropin 1

Stuart Sullivana,*, Catriona E. Thomsona,*, Douglas J. Lamontb, Matthew A. Jonesa and John M. Christiea,1

a Plant Science Group, Division of Biochemistry and Molecular Biology, Institute of Biomedical and Life Sciences, University of Glasgow, University Avenue, Glasgow, Scotland, UK
b FingerPrints Proteomics Facility, Post-Genomics and Molecular Interactions Centre, School of Life Sciences, MSI/WTB/CIR Complex, University of Dundee, Dundee, Scotland, UK

1 To whom correspondence should be addressed. E-mail J.Christie{at}bio.gla.ac.uk, fax +44 141 330 4447.

Phototropins (phot1 and phot2) are blue-light receptor kinases controlling a range of responses that optimize the photosynthetic efficiency of plants. Light sensing is mediated by two flavin-binding motifs, known as LOV1 and LOV2, located within the N-terminal region of the protein. Photoexcitation via LOV2 leads to activation of the C-terminal kinase domain and consequently receptor autophosphorylation. However, knowledge of the in-vivo phosphorylation sites for Arabidopsis phototropins is lacking and has impeded progress in elucidating the functional significance of receptor phosphorylation. We have purified phot1 from Arabidopsis and identified the in-vivo sites of receptor phosphorylation by liquid chromatography tandem mass spectrometry. Arabidopsis-derived phot1 binds flavin mononucleotide as chromophore and is phosphorylated at four major sites located upstream of LOV2 (Ser58, Ser85, Ser350, and Ser410), three of which are induced by blue light. Nevertheless, structure-function analysis indicates that the biological activity of phot1 can be attributed to a modular unit comprising the LOV2-kinase region of the protein. Thus, peptide regions upstream of LOV2, including the sites of receptor phosphorylation identified here, do not appear to be important for receptor signaling. By contrast, these regions may be necessary for maximizing stomatal performance and possibly light-induced relocalization of phot1.

* These authors contributed equally to the work.


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