Alternative translation initiation generates acyl-CoA synthetase 3 isoforms with heterogeneous amino termini

ACS3 is a recently identified acyl-CoA synthetase (ACS) isozyme that preferentially utilizes laurate, myristate, arachidonate, and eicosapentaenoate among saturated and unsaturated long chain fatty acids. The ACS3 purified from COS cells transfected with the ACS3 cDNA was separated by SDS-PAGE into two major forms of 79 and 80 kDa. We report here that alternative translation initiation from ACS3 mRNA gives rise to these two isoforms of ACS3. In vitro mutagenesis of the ACS3 cDNA revealed that the translation of the 80-kDa and 79-kDa isoforms started from the first and second in-frame AUGs, respectively. The two isoforms of ACS3 expressed in COS cells exhibited similar levels of ACS activities toward palmitate and myristate. Immunocytochemistry of intact COS cells transfected with various ACS3 expression vectors suggested that the two forms are localized in the extranuclear compartment, where they exhibit a reticular pattern. In rat cerebrum, the 80-kDa isoform of ACS3 was detected mainly in the microsomal fraction. Only a trace amount of the 79-kDa isoform was detected in rat cerebrum, whereas both forms were detected in rat glioma cell line KEG1 cells.