An N-terminal di-proline motif is essential for fatty acid–dependent degradation of Δ9-desaturase in Drosophila

Abstract

Edited by Dennis R. VoelkerThe ⌬9-fatty acid desaturase introduces a double bond at the ⌬9 position of the acyl moiety of acyl-CoA and regulates the cellular levels of unsaturated fatty acids. However, it is unclear how ⌬9-desaturase expression is regulated in response to changes in the levels of fatty acid desaturation. In this study, we found that the degradation of DESAT1, the sole ⌬9-desaturase in the Drosophila cell line S2, was significantly enhanced when the amounts of unsaturated acyl chains of membrane phospholipids were increased by supplementation with unsaturated fatty acids, such as oleic and linoleic acids. In contrast, inhibition of DESAT1 activity remarkably suppressed its degradation. Of note, removal of the DESAT1 N-terminal domain abolished the responsiveness of DESAT1 degradation to the level of fatty acid unsaturation. Further truncation and amino acid replacement analyses revealed that two sequential prolines, the second and third residues of DESAT1, were responsible for the unsaturated fatty acid-dependent degradation. Although degradation of mouse stearoyl-CoA desaturase 1 (SCD1) was unaffected by changes in fatty acid unsaturation, introduction of the N-terminal sequential proline residues into SCD1 conferred responsiveness to unsaturated fatty acid-dependent degradation. Furthermore, we also found that the Ca 2؉ -dependent cysteine protease calpain is involved in the sequential proline-dependent degradation of DESAT1. In light of these findings, we designated the sequential prolines at the second and third positions of DESAT1 as a "di-proline motif," which plays a crucial role in the regulation of ⌬9-desaturase expression in response to changes in the level of cellular unsaturated fatty acids.

Structure and Function of Δ9-Fatty Acid Desaturase

“…While the degradation rate of SCD1 protein is irrespective of the cellular levels of unsaturated fatty acids, 33) we recently reported that the expression level of Δ9-desaturase is posttranslationally regulated by the cellular fatty acid composition in Drosophila melanogaster. 34) Drosophila melanogaster is a model organism that provides advantages for the study of mechanisms underlying the expression and function of Δ9desaturase. 34) First, DESAT1 is the sole fatty acid desaturase that introduces a cis double bond into acyl chain of acyl-CoA in typical Drosophila cell lines such as S2 cells because another fatty acid desaturase, DESAT2, is not expressed.…”

Co-conspirators in a new mechanism for the degradation of Δ9-desaturase

“…However, none of the known regulatory mechanisms are directly responsive to changes in the levels of fatty acid desaturation among cellular lipids, raising the question as to how cells coordinate desaturase activity in response to changing environmental conditions. A new study by Murakami et al (2) provides one answer with the discovery of a fatty acid saturation-sensitive pathway for protein degradation in Drosophila that surprisingly depends on a di-proline motif in the N-terminal region of the desaturase.…”

“…In the study by Murakami et al (2), the authors focus on the Drosophila enzyme DESAT1, the only desaturase expressed in the S2 cell line. Using a polyclonal antibody raised against a C-terminal peptide from the enzyme, they measured DESAT1 levels upon treatment of cells with saturated, monounsaturated, or polyunsaturated fatty acids.…”

“…Thus, searching for mammalian regulatory pathways similar to the calpain-mediated degradation of ⌬9-desaturase could potentially lead to new therapeutic opportunities for decreasing SCD1 activity and help in the treatment of diseases of the metabolic syndrome. Top, putative location of the N-terminal di-proline motif in DESAT1, a ⌬9-desaturase expressed in Drosophila, identified by Murakami et al (2). Topology shown is modeled on the mouse enzyme (7).…”

“…Additional constructs with truncations at the N terminus or alanine mutations identified residues 1-6, and specifically Pro 2 and Pro 3 , as necessary and sufficient for the MUFAdependent degradation of DESAT1 (Fig. 1).…”