CHARACTERIZATION OF A STRUCTURALLY AND FUNCTIONALLY DIVERGED ACYL-ACYL CARRIER PROTEIN DESATURASE FROM MILKWEED SEED

A cDNA for a structurally variant acyl-acyl carrier protein (ACP) desa turase was isolated from milkweed (Asclepias syriaca) seed, a tissue e nriched in palmitoleic (16:1 Delta(9)) and cis-vaccenic (18:1 Delta(1 1)) acids. Extracts of Escherichia coli that express the milkweed cDNA catalyzed Delta(9) desaturation of acyl-ACP substrates, and the recom binant enzyme exhibited seven- to ten-fold greater specificity for pal mitoyl (16:0)-ACP and 30-fold greater specificity for myristoyl (14:0) -ACP than did known Delta(9)-stearoyl (18:0)-ACP desaturases. Like oth er variant acyl-ACP desaturases reported to date, the milkweed enzyme contains fewer amino acids near its N-terminus compared to previously characterized Delta(9)-18:0-ACP desaturases. Based on the activity of an N-terminal deletion mutant of a Delta(9)-18:0-ACP desaturase, this structural feature likely does not account for differences in substrat e specificities.