The Mycobacterium tuberculosis desaturase DesA1 (Rv0824c) is a Ca²⁺ binding protein

The hallmark feature of Mycobacterium tuberculosis (M.tb) the causative agent of human tuberculosis, is its complex lipid rich cell wall comprised primarily of mycolic acids, long chain fatty acids that play a key role in structural stability and permeability of the cell wall. In addition, they are involved in inhibiting phagosome-lysosome fusion and aid in granuloma formation during the pathogenic process. M.tb DesA1 is an essential acyl-acyl carrier protein desaturase predicted to catalyze the introduction of position specific double bonds during the biosynthesis of mycolic acids. This protein is one among three annotated desaturases (DesA1-3) in the M.tb genome but is unique in containing a βγ-crystallin Greek key signature motif, a well-characterized fold known to mediate Ca²⁺ binding in both prokaryotic and eukaryotic organisms. Using Isothermal Titration Calorimetry and ⁴⁵CaCl₂ overlay, we demonstrate that Ca²⁺ binds to DesA1. Spectroscopic measurements suggested that this binding induces changes in protein conformation but does not lead to significant alterations in the secondary structure of the protein, a feature common to several βγ-crystallins. An M. smegmatis strain over-expressing M.tb desA1 showed a Ca²⁺ dependent variation in surface phenotype, revealing a functional role for Ca²⁺in DesA1 activity. This study represents the first identification of a Ca²⁺ binding βγ-crystallin in M.tb, emphasizing the implicit role of Ca²⁺ in the pathogenesis of M.tb. - Highlights: • Mycobacterium tuberculosis DesA1 is an essential acyl-ACP desaturase. • DesA1 was identified to contain a βγ-crystallin Greek key signature motif. • Ca²⁺ binds to DesA1 with an affinity of 53 μM and induces changes in its conformation. • M. smegmatis overexpressing M.tb DesA1 shows a Ca²⁺ dependent phenotype. • Targetting the Ca²⁺ dependent function of DesA1 could be of therapeutic value.