Isoforms of acyl carrier protein involved in seed‐specific fatty acid synthesis

Summary Seeds ofCoriandrum sativum(coriander) andThunbergia alata(black‐eyed Susan vine) produce unusual monoenoic fatty acids which constitute over 80% of the total fatty acids of the seed oil. The initial step in the formation of these fatty acids is the desaturation of palmitoyl‐ACP (acyl carrier protein) at the D 4 or D 6 positions to produce D 4 ‐hexadecenoic acid (16 : 1 D4 ) or D 6 ‐hexadecenoic acid (16 : 1 D6 ), respectively. The involvement of specific forms of ACP in the production of these novel monoenoic fatty acids was studied. ACPs were partially purified from endosperm of coriander andT. alataand used to generate 3 H‐ and 14 C‐labelled palmitoyl‐ACP substrates. In competition assays with labelled palmitoyl‐ACP prepared from spinach (Spinacia oleracea), D 4 ‐acyl‐ACP desaturase activity was two‐ to threefold higher with coriander ACP than with spinach ACP. Similarly, theT. alataD 6 desaturase favouredT. alataACP over spinach ACP. A cDNA clone,Cs‐ACP‐I, encoding ACP was isolated from a coriander endosperm cDNA library.Cs‐ACP‐I mRNA was predominantly expressed in endosperm rather than leaves. TheCs‐ACP‐I mature protein was expressed inE. coliand co‐migrated on SDS–PAGE with the most abundant ACP expressed in endosperm tissues. Inin vitroD 4 ‐palmitoyl‐ACP desaturase assays, theCs‐ACP‐I expressed fromE. coliwas four‐ and 10‐fold more active than spinach ACP orE. coliACP, respectively, in the synthesis of D 4 ‐hexadecenoic acid from palmitoyl‐ACP. In contrast, D 9 ‐stearoyl‐ACP desaturase activity from coriander endosperm did not discriminate strongly between different ACP species. These results indicate that individual ACP isoforms are specifically involved in the biosynthesis of unusual seed fatty acids and further suggest that expression of multiple ACP isoforms may participate in determining the products of fatty acid biosynthesis.