Substrate-induced transmembrane signaling in the cobalamin transporter BtuB

Department of Microbiology, University of Virginia, Charlottesville, Virginia 22908, USA.
Nature Structural Biology 06/2003; 10(5):394-401. DOI: 10.1038/nsb914
Source: PubMed

ABSTRACT The outer membranes of Gram-negative bacteria possess transport proteins essential for uptake of scarce nutrients. In TonB-dependent transporters, a conserved sequence of seven residues, the Ton box, faces the periplasm and interacts with the inner membrane TonB protein to energize an active transport cycle. A critical mechanistic step is the structural change in the Ton box of the transporter upon substrate binding; this essential transmembrane signaling event increases the affinity of the transporter for TonB and enables active transport to proceed. We have solved crystal structures of BtuB, the outer membrane cobalamin transporter from Escherichia coli, in the absence and presence of cyanocobalamin (vitamin B(12)). In these structures, the Ton box is ordered and undergoes a conformational change in the presence of bound substrate. Calcium has been implicated as a necessary factor for the high-affinity binding (K(d) approximately 0.3 nM) of cyanocobalamin to BtuB. We observe two bound calcium ions that order three extracellular loops of BtuB, thus providing a direct (and unusual) structural role for calcium.

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    • "The TonB-dependent vitamin B 12 importer BtuCED (TCID 3.A.1.13) has been identified in E. coli (de Veaux et al. 1986) and imports vitamin B 12 in corrinoids (Borths et al. 2002; Chimento et al. 2003; Locher et al. 2002) to the periplasmic binding protein BtuF (Hvorup et al. 2007). However, the requirement of a vitamin B 12 ABC permease for full virulence has not yet been established in an in vivo infection model. "
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    ABSTRACT: The ATP-binding cassette transporter superfamily is present in all three domains of life. This ubiquitous class of integral membrane proteins have diverse biological functions, but their fundamental role involves the unidirectional translocation of compounds across cellular membranes in an ATP coupled process. The importance of this class of proteins in eukaryotic systems is well established as typified by their association with genetic diseases and roles in the multi-drug resistance of cancer. In stark contrast, the ABC transporters of prokaryotes have not been exhaustively investigated due to the sheer number of different roles and organisms in which they function. In this review, we examine the breadth of functions associated with microbial ABC transporters in the context of their contribution to bacterial pathogenicity and virulence.
    Protoplasma 01/2012; 249(4):919-42. DOI:10.1007/s00709-011-0360-8 · 2.65 Impact Factor
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    • "). These proteins have been analyzed in great detail and the crystal structures of BtuB (Chimento et al., 2003), the SBP BtuF (Borths et al., 2002), the complex of BtuC (TMD) and BtuD (NBD) (Locher et al., 2002) and the BtuCD-BtuF (Hvorup et al., 2007) holotransporter have been determined. Related ABC transporters are widespread among bacteria and archaea, and the respective genes are regulated by adenosylcobalamin-dependent riboswitch (B12 element) in most cases (Rodionov et al., 2003). "
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    ABSTRACT: Since their discovery in the 1960s as 'osmotic shock-sensitive' transporters, a plethora of so-called binding protein-dependent (canonical) ATP-binding cassette (ABC) importers has been identified in bacteria and archaea. Their cellular functions go far beyond the uptake of nutrients. Canonical ABC importers play important roles in the maintenance of cell integrity, responses to environmental stresses, cell-to-cell communication and cell differentiation and in pathogenicity. A new class of abundant micronutrient importers, the 'energy-coupling factor' (ECF) transporters, was originally identified by functional genomics. ABC ATPases are an integral part of both canonical ABC and ECF importers. Fundamental differences include the modular architecture and the independence of ECF systems of extracytoplasmic solute-binding proteins. This review describes the roles of both types of transporters in diverse physiological processes including pathogenesis, points to the differences in modular assembly and depicts their common traits.
    FEMS microbiology reviews 04/2010; 35(1):3-67. DOI:10.1111/j.1574-6976.2010.00230.x · 13.24 Impact Factor
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    • "The inner membrane protein TonB is required for the activity of a number of outer membrane transporters, including FepA (Buchanan et al., 1999), BtuB (Chimento et al., 2003) and FhuA (Killmann & Braun, 1994). However, infection of T5 (Killmann & Braun, 1994) or BF23 (Rabsch et al., 2007) is not dependent on TonB, although that of phage H8 is TonB dependent (Rabsch et al., 2007). "
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    ABSTRACT: The virulent bacteriophage EPS7 active against a number of Salmonella serovar and Escherichia coli strains, isolated from the local sewage in Korea, belongs to the family Siphoviridae. The ESP7 genome constitutes a linear double-stranded DNA of 111 382 bp. DNA sequencing and genomic analysis of EPS7 showed that it belongs to the phage T5 family. We identified the EPS7 genes involved in DNA repair, replication, viral structure and bacterial lysis by comparing the EPS7 genome with that of T5. In contrast, the tail genes encoding for putative host receptor-binding protein and the putative receptor-blocking lipoprotein precursor of EPS7 exhibit high homologies with the corresponding gene products of BF23, another member of the T5-family. BF23 binds to BtuB, a surface receptor in the host and involved in vitamin B12 uptake, but its infection is independent of TonB. By constructing a series of deletion mutants in Salmonella and in E. coli and studying phage infection in the mutant hosts, we showed that BtuB is also the host receptor of the phage EPS7. Whether EPS7 infection depends on TonB needs to be further studied.
    FEMS Microbiology Letters 12/2008; 289(2):202-9. DOI:10.1111/j.1574-6968.2008.01397.x · 2.12 Impact Factor
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