TARGETING AND TOPOLOGY IN THE MEMBRANE OF PLANT 3-HYDROXY-3-METHYLGLUTARYL COENZYME-A REDUCTASE

The enzyme 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGR) cata lyzes the synthesis of mevalonate. This is the first committed step of isoprenoid biosynthesis. A common feature of all known plant HMGR iso forms is the presence of two highly conserved hydrophobic sequences in the N-terminal quarter of the protein. Using an in vitro system, we s howed that the two hydrophobic sequences of Arabidopsis HMGR1S functio n as internal signal sequences. Specific recognition of these sequence s by the signal recognition particle mediates the targeting of the pro tein to microsomes derived from the endoplasmic reticulum. Arabidopsis HMGR is inserted into the microsomal membrane, and the two hydrophobi c 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 s equence is exposed to the lumen. Our results suggest that the plant HM GR isoforms known to date are primarily targeted to the endoplasmic re ticulum and have the same topology in the membrane. This reinforces th e hypothesis that mevalonate is synthesized only in the cytosol. The p ossibility that plant HMGRs might be located in different regions of t he endomembrane system is discussed.