Ra. Larsen et al., THE CONSERVED PROLINE-RICH MOTIF IS NOT ESSENTIAL FOR ENERGY TRANSDUCTION BY ESCHERICHIA-COLI TONB PROTEIN, Molecular microbiology, 10(5), 1993, pp. 943-953
TonB protein functions as an energy transducer, coupling cytoplasmic m
embrane electrochemical potential to the active transport of vitamin B
-12 and Fe(III)-siderophore complexes across the outer membrane of Esc
herichia coli and other Gram-negative bacteria. Accumulated evidence i
ndicates that TonB is anchored in the cytoplasm, but spans the peripla
smic space to interact physically with outer membrane receptors. ft ha
s been presumed that this ability is caused by a conserved (Glu-Pro)n-
(Lys-Pro)m repeat motif, predicted to assume a rigid, linear conformat
ion of sufficient length to reach the outer membrane. Based on in vitr
o studies with synthetic peptides and purified FhuA outer membrane rec
eptor, it has been suggested that this region contains a site that dir
ectly binds outer membrane receptors and is essential for energy trans
duction. We have found a TonB lacking the (Glu-Pro)n-(Lys-Pro)m repeat
motif (TonBDELTA(66-100)). TonBDELTA(66-100) is fully capable of irre
versible phi80 adsorption, except under physiological circumstances wh
ere the periplasmic space is expanded. Based on the ability of TonBDEL
TA(66-1 00) to interact with outer membrane receptors and components o
f the energy transduction apparatus under normal physiological conditi
ons, it is evident that the TonB prolinerich region has no role in ene
rgy transduction other than to provide a physical extension sufficient
to reach the outer membrane.