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

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Abstract
Seeds of Coriandrum sativum (coriander) and Thunbergia 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 D4 or D6 positions to produce D4-hexadecenoic acid (16 : 1D4) or D6-hexadecenoic acid (16 : 1D6), 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 and T. alata and used to generate 3H- and 14C-labelled palmitoyl-ACP substrates. In competition assays with labelled palmitoyl-ACP prepared from spinach ( Spinacia oleracea ), D4-acyl-ACP desaturase activity was two- to threefold higher with coriander ACP than with spinach ACP. Similarly, the T. alata D6 desaturase favoured T. alata ACP 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. The Cs -ACP-I mature protein was expressed in E. coli and co-migrated on SDS–PAGE with the most abundant ACP expressed in endosperm tissues. In in vitro D4-palmitoyl-ACP desaturase assays, the Cs -ACP-I expressed from E. coli was four- and 10-fold more active than spinach ACP or E. coli ACP, respectively, in the synthesis of D4-hexadecenoic acid from palmitoyl-ACP. In contrast, D9-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.