Ser/Thr phosphorylation regulates the Fatty Acyl-AMP Ligase activity of FadD32, an essential enzyme in mycolic acid biosynthesis

Abstract Mycolic acids are essential components of the mycobacterial cell envelope and their biosynthetic pathway is a well-known source of antituberculous drug targets. Among the promising new targets in the pathway, FadD32 is an essential enzyme required for the activation of the long meromycolic chain of mycolic acids and essential for mycobacterial growth. Following the in-depth biochemical, biophysical and structural characterization of FadD32, we investigated its putative regulation via post-translational modifications (PTM). Comparison of the fatty acyl-AMP ligase activity between phosphorylated and dephosphorylated FadD32 isoforms showed that the native protein is phosphorylated by Serine/Threonine Protein Kinases (STPKs), and that this phosphorylation induced significant loss of activity. Mass spectrometry analysis of the native protein confirmed the PTM and identified Thr552 as the phosphosite. Phosphoablative and phosphomimetic FadD32 mutant proteins confirmed both the position and the importance of the modification and its correlation with the negative regulation of FadD32 activity. Investigation of the mycolic acid condensation reaction catalyzed by Pks13, involving FadD32 as a partner, showed that FadD32 phosphorylation also impacts the condensation activity. Altogether our results bring to light FadD32 phosphorylation by STPKs and its correlation with the enzyme negative regulation, thus shedding a new horizon on the mycolic acid biosynthesis modulation and possible inhibition strategies for this promising drug target.