Transport, Compartmentation, and Metabolism of Homoserine in
Higher Plant Cells
Carbon-13- and Phosphorus-31-Nuclear Magnetic Resonance Studies
Serge Aubert,
Gilles Curien,
Richard Bligny*,
Elisabeth Gout, and
Roland Douce
Laboratoire de Physiologie Cellulaire Végétale,
Unité de Recherche Associée 576 Centre National de la
Recherche Scientifique (S.A., R.B., R.D.); Laboratoire de
Résonance Magnétique en Biologie Métabolique,
Département de Biologie Moléculaire et Structurale,
Commissariat à l'Energie Atomique, 17 rue des Martyrs, 38054 Grenoble cédex 9, France (E.G.); and Laboratoire Mixte Centre
National de la Recherche Scientifique/Rhône-Poulenc Unité
Mixte de Recherche 41, Rhône-Poulenc Agrochimie, 14-20 rue
Pierre Baizet, 69263 Lyon, France (G.C., R.D.)
The transport, compartmentation, and
metabolism of homoserine was characterized in two strains of
meristematic higher plant cells, the dicotyledonous sycamore
(Acer pseudoplatanus) and the monocotyledonous weed
Echinochloa colonum. Homoserine is an intermediate in
the synthesis of the aspartate-derived amino acids methionine, threonine (Thr), and isoleucine. Using 13C-nuclear magnetic
resonance, we showed that homoserine actively entered the cells via a
high-affinity proton-symport carrier (Km approximately 50-60 µm) at the maximum rate of 8 ± 0.5 µmol h
1 g1 cell wet weight, and in
competition with serine or Thr. We could visualize the compartmentation
of homoserine, and observed that it accumulated at a concentration 4 to
5 times higher in the cytoplasm than in the large vacuolar compartment.
31P-nuclear magnetic resonance permitted us to analyze the
phosphorylation of homoserine. When sycamore cells were incubated with
100 µm homoserine, phosphohomoserine steadily accumulated
in the cytoplasmic compartment over 24 h at the constant rate of 0.7 µmol h1 g1 cell wet weight, indicating
that homoserine kinase was not inhibited in vivo by its product,
phosphohomoserine. The rate of metabolism of phosphohomoserine was much
lower (0.06 µmol h1 g1 cell wet weight)
and essentially sustained Thr accumulation. Similarly, homoserine was
actively incorporated by E. colonum cells. However, in
contrast to what was seen in sycamore cells, large accumulations of Thr
were observed, whereas the intracellular concentration of homoserine
remained low, and phosphohomoserine did not accumulate. These
differences with sycamore cells were attributed to the presence of a
higher Thr synthase activity in this strain of monocot cells.
*
Corresponding author; e-mail bligny{at}dsvgre.cea.fr; fax
33-4-76-88-50-91.
Plant Physiol. (1998) 116: 547-557
Copyright Clearance Center: 0032-0889/98/116/0547/11
© 1998 American Society of Plant Physiologists
