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. 2020 Jun 10;295(30):10456–10467. doi: 10.1074/jbc.RA120.014081

Figure 1.

Figure 1.

Heme-dependent cell surface signaling and transport. A, schematic of ECF factor activation and heme transport. 1, in the absence of heme, the HasR receptor N-terminal plug occludes the periplasm from the extracellular environment. 2, holo-HasAp, on interaction with HasR, releases heme, triggering conformational rearrangement of the N-terminal plug driving interaction with the anti-σ factor HasS. This event triggers the release of the σ factor HasI, recruitment of the core RNA polymerase, and transcriptional activation of the has operon. 3, TonB on interaction with the heme-loaded HasR partially displaces the N-terminal plug, facilitating heme transport to the periplasm. Subsequently, heme is sequestered by PhuT and translocated to the cytoplasm by PhuUV, where it is trafficked by PhuS to HemO for degradation and iron release. B, structure of the S. marcescens holo-HasAp-HasR complex showing HasAp (light blue) and HasR with the N-terminal plug (magenta) and β-barrel (gray). The heme (green) and heme-coordinating residues His-189 (magenta), His-603 on FRAP/PNPNL loop (green), and Ile-671 (dark blue) are shown as sticks. The numbers in parentheses represent the corresponding residues in P. aeruginosa HasR. The image was generated from PDB file 3CSL in PyMOL (30).