Ratio of active to inactive forms of acyl carrier protein in Escherichia coli.

Acyl carrier protein (ACP) functions as a cofactor in fatty acid biosynthesis due to the covalent linkage of an acyl moiety to its 4'-phosphopantetheine prosthetic group. This prosthetic group undergoes turnover in vivo and since the apoprotein is functionally inactive, the interconversion between ACP and apo-ACP has been considered as a possible regulatory point in lipid biosynthesis. To investigate this possibility, the ratio of ACP to apo-ACP was measured in Escherichia coli. An apo-ACP standard was synthesized using [ACP] phosphodiesterase (EC 3.1.4.14) and could be clearly separated from ACP by conformationally sensitive gel electrophoresis, thus providing a reliable assay for the presence of these two species. Antibodies specific for ACP were purified from rabbit serum on an ACP-Sepharose column and subsequently used to synthesize an immunoaffinity column. Chromatography of leucine-labeled cell extracts on this support resulted in the specific binding of ACP, but apo-ACP was not detected in either logarithmically growing or stationary phase cells, although both ACP species bound to the purified anti-ACP IgG. Apo-ACP was not detected as an intermediate in ACP biosynthesis, suggesting that apo-ACP is rapidly converted to ACP following translation. CoA is the biosynthetic precursor to the ACP prosthetic group, but apo-ACP did not accumulate when the intracellular CoA concentration was severely depressed in strain SJ16 (panD), a beta-alanine auxotroph. Strain MP4 (acpS) is conditionally defective in [ACP]synthase (EC 2.7.8.7) and apo-ACP was the predominant form of ACP synthesized in this strain under nonpermissive conditions. Even under conditions that permitted growth, apo-ACP comprised 70% of the total ACP pool in strain MP4. Strain MP4 possessed a phospholipid to protein ratio within the normal range, suggesting that the ratio of ACP to apo-ACP can be significantly altered without affecting total lipid content. Thus, it appears that the prosthetic group turnover cycle maintains all of the ACP in an active form in vivo and a regulatory role for the ACP/apo-ACP ratio seems doubtful.