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THE PLANT CELL, Vol 7, Issue 12 2163-2174, Copyright © 1995 by American Society of Plant Biologists
Targeting and Topology in the Membrane of Plant 3-Hydroxy-3-Methylglutaryl Coenzyme A Reductase
N. Campos and A. Boronat
Departament de Bioquimica i Biologia Molecular, Facultat de Quimica, Universitat de Barcelona, C/ Marti i Franques 1, 08028 Barcelona, Spain
The enzyme 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGR) catalyzes
the synthesis of mevalonate. This is the first committed step of isoprenoid
biosynthesis. A common feature of all known plant HMGR isoforms is the
presence of two highly conserved hydrophobic sequences in the N-terminal
quarter of the protein. Using an in vitro system, we showed that the two
hydrophobic sequences of Arabidopsis HMGR1S function as internal signal
sequences. Specific recognition of these sequences by the signal
recognition particle mediates the targeting of the protein to microsomes
derived from the endoplasmic reticulum. Arabidopsis HMGR is inserted into
the microsomal membrane, and the two hydrophobic sequences become
membrane-spanning segments. The N-terminal end and the C-terminal catalytic
domain of Arabidopsis HMGR are positioned on the cytosolic side of the
membrane, whereas only a short hydrophilic sequence is exposed to the
lumen. Our results suggest that the plant HMGR isoforms known to date are
primarily targeted to the endoplasmic reticulum and have the same topology
in the membrane. This reinforces the hypothesis that mevalonate is
synthesized only in the cytosol. The possibility that plant HMGRs might be
located in different regions of the endomembrane system is discussed.
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