TipA, a Bdellovibrio bacteriovorus BPI-like double-TULIP is opened by its adapter protein, TipB

Bdellovibrio bacteriovorus is a predatory bacterium that invades the periplasm of other Gram-negative bacteria and liberates prey biomolecules for replication. Bdellovibrio has a wealth of genes that encode unique proteins to enable this lifestyle. Using a series of x-ray structures, we show that the operonal pairing of bd2538 and bd2539, encode a double TULIP (tubular lipid binding protein) invasion protein A (TipA) and a small beta sandwich (TipB), respectively. TipA has a specialised N-terminal TULIP domain with a beta hairpin and helix that mediate homodimerisation through hairpin interdigitation. This dimerisation creates a large, continuous, enclosed lumen that we demonstrate to contain multiple lipid molecules. In addition, we show that TipB functions as a small adapter protein that binds to TipA using specialised loops that bury into the TipA hydrophobic core. This binding forms a clamp on the edge of the N-terminal beta sheet and induces a large 20 [A] conformational change, opening the TULIP fold to create an accessible interior. This study presents the first structural characterisation of lipid binding proteins in Bdellovibrio, and the first example of conformational change in TULIPs mediated by an adaptor protein. Synopsis O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=186 SRC="FIGDIR/small/675085v2_ufig1.gif" ALT="Figure 1"> View larger version (47K): org.highwire.dtl.DTLVardef@3ef907org.highwire.dtl.DTLVardef@4f555dorg.highwire.dtl.DTLVardef@6d560forg.highwire.dtl.DTLVardef@2cfe49_HPS_FORMAT_FIGEXP M_FIG C_FIG Bdellovibrio bacteriovorus is a predatory bacterium that invades the periplasm of other Gram-negative bacteria in order to consume them from within. To enable this lifestyle, it has an arsenal of predation-associated genes, including the operonal pair bd2538 and bd2539. We use structural and biophysical techniques to characterise two proteins produced by these genes, TipA and TipB. O_LITipA is a double tubular lipid binding protein (TULIP) that forms a homodimer that sequesters lipids in its lumen and binds to lipid bilayers C_LIO_LITipB is a small beta sandwich that interacts with TipA via two specialised loops C_LIO_LIInteraction of TipA with TipB opens the TULIP fold of TipA C_LI