Tatjana Lapina1, Karl Forchhammer2 and Elena Ermilova1
1 Department of Comparative Biological Sciences, School of Veterinary Medicine, Louisiana State University, Baton Rouge LA, USA
2 Institute of Cytology, Russian Academy of Sciences, St. Petersburg, Russia
3 Department of Pathobiological Sciences, School of Veterinary Medicine, Louisiana State University, Baton Rouge LA, USA
| Submitted: September 17, 2016 | Accepted: October 20, 2016 |
Summary
Homoserine kinase (EC 2.7.1.39, HSK) produces O-phospho-L-homoserine, a
branch point intermediate in the pathways for methionine (Met) and for threonine
(Thr)/isoleucine (Ile) in photosynthetic eukaryotes. Although HSKs have been
described in bacteria, archaea and higher plants, no HSK homolog has so far been
characterized in green algae.
In the unicellular green alga Chlamydomonas reinhardtii, the HSK1 is encoded
by a nuclear gene CrHSK1. From the analysis of the localizations demonstrated
for homologous proteins in higher plants Chlamydomonas HSK is predicted as
chloroplast protein. The N-terminal part of CrHSK1 until residue alanine 31
is predicted to encode a chloroplast transit peptide that is cleaved in the mature
protein. The C. reinhardtii HSK1 (CrHSK1) was cloned and overexpressed with
a C-terminal-fused His6-tag. Gel filtration showed the oligomeric structure of
CrHSK1 to be a homodimer. CrHSK1 has an apparent Km for L-homoserine of
0.107 mM and for ATP of 0.2 mM and it required K+ and Mg2+ for activity. CrHSK1
appears to be insensitive to the substrate and end-products of the aspartate pathway it
serves. In C. reinhardtii, CrHSK1 transcript levels are induced in response to peroxide
treatment. These results suggest that CrHSK1 might be implicated in oxidative stress
response. Characterization of HSK in the green alga provides a framework for a
more complete understanding of the regulation of this highly conserved enzyme.
Key words: aspartate pathway, Chlamydomonas reinhardtii, gene expression,
homoserine kinase, oxidative stress
Address for correspondence: Elena Ermilova. Lab. Adaptation in Microorganisms, Biological Faculty of
Saint-Petersburg State University, Universitetskaya emb. 7/9, 199034 Saint-Petersburg, Russia;
e-mail: e.ermilova@spbu.ru
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