Abstract
The very long chain fatty acids are crucial building blocks of essential lipids, most notably the sphingolipids. These elongated fatty acids are synthesized by a system of enzymes that are organized in a complex within the endoplasmic reticulum membrane. Although several of the components of the elongase complex have recently been identified, little is known about how these proteins are organized within the membrane or about how they interact with one another during fatty acid elongation. In this study the topology of Tsc13p, the enoyl reductase of the elongase system, was investigated. The N and C termini of Tsc13p reside in the cytoplasm, and six putative membrane-spanning domains were identified by insertion of glycosylation and factor Xa cleavage sites at various positions. The N-terminal domain including the first membrane-spanning segment contains sufficient information for targeting to the endoplasmic reticulum membrane. Studies of the Arabidopsis Tsc13p protein revealed a similar topology. Highly conserved domains of the Tsc13p proteins that are likely to be important for enzymatic activity lie on the cytosolic face of the endoplasmic reticulum, possibly partially embedded within the membrane.
Highlights
Cytosolic fatty acid synthases catalyze the de novo synthesis of the 16 or 18 carbons containing fatty acids that are further elongated to very long chain fatty acids by a microsomal enzyme system, the elongase
Because the exact features of membrane proteins that dictate how they insert into membranes are not fully understood, the available algorithms for predicting membrane topologies serve only as guides for the actual topology
We evaluated the topology of the Arabidopsis Tsc13p ortholog and find that it has a similar membrane topology to that of the yeast Tsc13p
Summary
Cytosolic fatty acid synthases catalyze the de novo synthesis of the 16 or 18 carbons containing fatty acids that are further elongated to very long chain fatty acids by a microsomal enzyme system, the elongase. The first predicted TMD lies between residues 86 and 110; the lack of modification of the potential glycosylation site at 38 is consistent with the protease sensitivity results and provides further evidence that the N terminus of Tsc13p is cytosolic.
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