A novel function of yeast fatty acid synthase

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Abstract
The prosthetic group of yeast fatty acid synthase (FAS), 4′‐phosphopantetheine, is covalently linked to Ser180 of subunit α. It originates from coenzyme A and is transferred to the enzyme by a specific phosphopantetheine:protein transferase (PPTase). The present study demonstrates that the FAS‐activating PPTase of yeast represents a distinct catalytic domain of the FAS complex and resides within the C‐terminal portion of subunit α. The autoactivation capacity of yeast FAS became evident from in vitro pantetheinylation studies using purified apo‐FAS preparations. These were readily converted to pantetheinylated holo‐FAS simply upon addition of free coenzyme A. Pantetheinylation‐competent apo‐FAS was prepared in vitro by constructing hybrid oligomers containing α‐subunits from two different pantetheine‐less FAS‐mutants. The respective mutants were selected according to their ability to complement each other, in vivo. In vitro formation of hybrid apo‐FAS complexes was achieved by dimethylmaleic anhydride (DMMA) ‐induced reversible dissociation of mixtures of the two constituent mutant enzymes. This treatment was both necessary and sufficient to produce pantetheinylation‐competent apo‐FAS. Specific FAS activities were comparable independent of whether the apo‐enzymes were pantetheinylated in vivo or in vitro. Apart from the induction of overall FAS activity, incorporation of phosphopantetheine into apo‐FAS was also demonstrated by the use of 3H‐labelled coenzyme A, leading to the formation of radioactively labelled FAS. It is concluded that pantetheinylation of yeast FAS is performed by an intrinsic catalytic activity of the apo‐enzyme proper. The endogenous PPTase acts in trans between different subunits α in the α6β6 oligomer. The self‐pantetheinylation of yeast FAS represents the first example of an apo‐enzyme being capable of post‐translational autoactivitation.