The conserved modular elements of the acyl carrier proteins of lipid synthesis are only partially interchangeable.

Prior work showed that expression of acyl carrier proteins (ACPs) of a diverse set of bacteria replaced the function of Escherichia coli ACP in lipid biosynthesis. However, the AcpAs of Lactococcus lactis and Enterococcus faecalis were inactive. Both failed to support growth of an E. coli acpP mutant strain. This defect seemed likely because of the helix II sequences of the two AcpAs, which differed markedly from those of the proteins that supported growth. To test this premise, chimeric ACPs were constructed in which L. lactis helix II replaced helix II of E. coli AcpP and vice versa. Expression of the AcpP protein L. lactis AcpA helix II allowed weak growth, whereas the L. lactis AcpA-derived protein that contained E. coli AcpP helix II failed to support growth of the E. coli mutant strain. Replacement of the L. lactis AcpA helix II residues in this protein showed that substitution of valine for the phenylalanine residue four residues downstream of the phosphopanthetheine-modified serine gave robust growth and allowed modification by the endogenous AcpS phosphopantetheinyl transferase (rather than the promiscuous Sfp transferase required to modify the L. lactis AcpA and the chimera of L. lactis AcpA helix II in AcpP). Further chimera constructs showed that the lack of function of the L. lactis AcpA-derived protein containing E. coli AcpP helix II was due to incompatibility of L. lactis AcpA helix I with the downstream elements of AcpP. Therefore, the origins of ACP incompatibility can reside in either helix I or in helix II.