3NJT: Crystal Structure Of The R450a Mutant Of The Membrane Protein Fhac

In Gram-negative bacteria, the two-partner secretion pathway mediates the secretion of TpsA proteins with various functions. TpsB transporters specifically recognize their TpsA partners in the periplasm and mediate their transport through a hydrophilic channel. The filamentous haemagglutinin adhesin (FHA)/FhaC pair represents a model two-partner secretion system, with the structure of the TpsB transporter FhaC providing the bases to decipher the mechanism of action of these proteins. FhaC is composed of a beta-barrel preceded by two periplasmic polypeptide-transport-associated (POTRA) domains in tandem. The barrel is occluded by an N-terminal helix and an extracellular loop, L6, folded back into the FhaC channel. In this article, we describe a functionally important motif of FhaC. The VRGY tetrad is highly conserved in the TpsB family and, in FhaC, it is located at the tip of L6 reaching the periplasm. Replacement by Ala of the invariant Arg dramatically affects the secretion efficiency, although the structure of FhaC and its channel properties remain unaffected. This substitution affects the secretion mechanism at a step beyond the initial TpsA-TpsB interaction. Replacement of the conserved Tyr affects the channel properties, but not the secretion activity, suggesting that this residue stabilizes the loop in the resting conformation of FhaC. Thus, the conserved motif at the tip of L6 represents an important piece of two-partner secretion machinery. This motif is conserved in a predicted loop between two beta-barrel strands in more distant relatives of FhaC involved in protein transport across or assembly into the outer membranes of bacteria and organelles, suggesting a conserved function in the molecular mechanism of transport.
PDB ID: 3NJTDownload
MMDB ID: 85913
PDB Deposition Date: 2010/6/18
Updated in MMDB: 2010/11
Experimental Method:
x-ray diffraction
Resolution: 3.5  Å
Source Organism:
Similar Structures:
Biological Unit for 3NJT: monomeric; determined by author and by software (PISA)
Molecular Components in 3NJT
Label Count Molecule
Protein (1 molecule)
Filamentous Hemagglutinin Transporter Protein Fhac(Gene symbol: fhaC)
Molecule annotation
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Citing MMDB