Mass spectrometric characterization of oat phytochrome A: isoforms and posttranslational modifications

Protein Sci. 1999 May;8(5):1032-44. doi: 10.1110/ps.8.5.1032.


At least four mRNAs for oat phytochrome A (phyA) are present in etiolated oat tissue. The complete amino acid sequences of two phyA isoforms (A3 and A4) and the N-terminal amino acid sequence of a third isoform (A5) were deduced from cDNA sequencing (Hershey et al., 1985). In the present study, heterogeneity of phyA on a protein level was studied by tryptic mapping using electrospray ionization mass-spectrometry (ESIMS). The total tryptic digest of iodoacetamide-modified phyA was fractionated by gel filtration chromatography followed by reversed-phase high-performance liquid chromatography. ESIMS was used to identify peptides. Amino acid sequences of the peptides were confirmed or determined by collision-induced dissociation mass spectrometry (CID MS), MS/MS, or by subdigestion of the tryptic peptides followed by ESIMS analysis. More than 97% of the phyA3 sequence (1,128 amino acid residues) was determined in the present study. Mass-spectrometric analysis of peptides unique to each form showed that phyA purified from etiolated oat seedling is represented by three isoforms A5, A3, and A4, with ratio 3.4:2.3:1.0. Possible light-induced changes in phytochrome in vivo phosphorylation site at Ser7 (Lapko VN et al., 1997, Biochemistry 36:10595-10599) as well at Ser17 and Ser598 (known as in vitro phosphorylation sites) were also analyzed. The extent of phosphorylation at Ser7 appears to be the same for phyA isolated from dark-grown and red-light illuminated seedlings. In addition to Ser7, Ser598 was identified as an in vivo phosphorylation site in oat phyA. Ser598 phosphorylation was found only in phyA from the red light-treated seedlings, suggesting that the protein phosphorylation plays a functional role in the phytochrome A-mediated light-signal transduction.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Amino Acid Sequence
  • Avena / chemistry*
  • Chromatography, Gel
  • Mass Spectrometry*
  • Models, Biological
  • Molecular Sequence Data
  • Peptide Mapping
  • Phosphorylation
  • Phytochrome / chemistry*
  • Phytochrome A
  • Protein Isoforms*
  • Protein Processing, Post-Translational*


  • Phytochrome A
  • Protein Isoforms
  • Phytochrome