1 research outputs found
Identification of a Positively Charged Platform in <i>Staphylococcus aureus</i> HtsA That Is Essential for Ferric Staphyloferrin A Transport
In response to iron starvation, <i>Staphylococcus aureus</i> secretes both staphyloferrin A and
staphyloferrin B, which are high-affinity
iron-chelating molecules. The structures of both HtsA and SirA, the
ferric-staphyloferrin A [FeĀ(III)-SA] and ferric-staphyloferrin B [FeĀ(III)-SB]
receptors, respectively, have recently been determined. The structure
of HtsA identifies a novel form of ligand entrapment composed of many
positively charged residues. Through ionic interactions, the binding
pocket appears highly adapted for the binding of the highly anionic
siderophore SA. However, biological validation of the importance of
the nine SA-interacting residues (six arginines, one tyrosine, one
histidine, and one lysine) has not been previously performed. Here,
we mutated each of the FeĀ(III)-SA-interacting residues in HtsA and
found that substitutions R104A, R126A, H209A, R306A, and R306K resulted
in a reduction of binding affinity of HtsA for FeĀ(III)-SA. While mutation
of almost all proposed ligand-interacting residues decreased the ability
of <i>S. aureus</i> cells to transport <sup>55</sup>FeĀ(III)-SA, <i>S. aureus</i> expressing HtsA R104A, R126A, R306A, and R306K
showed the greatest transport defects and were incapable of growth
in iron-restricted growth media in a SA-dependent manner. These three
residues cluster together and, relative to other residues in the binding
pocket, move very little between the apo and closed holo structures.
Their essentiality for receptor function, together with structural
information, suggests that they form a positively charged platform
that is required for initial contact with the terminal carboxyl groups
of the two citrates in the FeĀ(III)-SA complex. This is a likely mechanism
by which HtsA discerns iron-bound SA from iron-free SA