C-lobe and N-lobe beta barrels of 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. This domain family is found in C and N lobe eight stranded beta barrel region of TbpB proteins. The eight-stranded barrel domains in N and C lobe draw comparisons to eight-stranded beta barrel outer-membrane protein W (OmpW). However, the barrel domains of TbpB have the hydrophobic residues line the inner surface of the beta barrels to create a stable hydrophobic core.