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N-Lobe handle Tf-binding protein B Bacterial lipoproteins represent a large group of specialized membrane proteins that perform a variety of functions including maintenance and stabilization of the cell envelope, protein targeting and transit to the outer membrane, membrane biogenesis, and cell adherence. Pathogenic Gram-negative bacteria within the Neisseriaceae and Pasteurellaceae families rely on a specialized uptake system, characterized by an essential surface receptor complex that acquires iron from host transferrin (Tf) and transports the iron across the outer membrane. They have an iron uptake system composed of surface exposed lipoprotein, Tf-binding protein B (TbpB), and an integral outer-membrane protein, Tf-binding protein A (TbpA), that together function to extract iron from the host iron binding glycoprotein (Tf). TbpB is a bilobed (N and C lobe) lipid-anchored protein with each lobe consisting of an eight-stranded beta barrel flanked by a 'handle' domain made up of four (N lobe) or eight (C lobe) beta strands. TbpB extends from the outer membrane surface by virtue of an N-terminal peptide region that is anchored to the outer membrane by fatty acyl chains on the N-terminal cysteine and is involved in the initial capture of iron-loaded Tf. The 4-residue conserved LSAC motif found at the amino terminus of TbpB represents a prototypical lipobox, with the cysteine residue serving as the first amino acid in the mature protein which is subsequently modified by the addition of a diacyl glycerol. A second conserved motif of interest is located two amino acids downstream of the LSAC site. This region consists of four glycine residues in tandem. Deletion of the conserved polyglycine motif has significant negative effects on growth in certain conditions, while mutational analysis revealed that the LSAC motif constituting the lipobox of TbpB is necessary for lipidation and hence tethering of TbpB to the bacterial surface. This domain family is found on the N-terminal region of TbpB proteins, which comprises the N lobe handle consisting of a four-stranded antiparallel beta sheets held together by a short surface-exposed alpha helix. Tf-binding activity primarily resides in the TbpB N lobe. |
Jiyao W et al.(2023). "The conserved domain database in 2023.",