J. Biol. Chem., Vol. 261, Issue 2, 622-626, 01, 1986
The role of the maltodextrin-binding site in determining the transport properties of the LamB protein
T Nakae, J Ishii and T Ferenci
We have examined by the liposome swelling technique the permeability
properties of the modified LamB proteins isolated from mutants of
Escherichia coli K12 with altered affinities toward starch and/or maltose
(Ferenci, T., and Lee, K-S. (1982) J. Mol. Biol. 160, 431-444). The results
revealed the following. A mutant strain exhibiting a markedly lowered
affinity toward starch produced a LamB protein that has lost the ability to
permeate longer maltodextrins. This protein retained a nonspecific pore for
a wide variety of small sugars. A mutant strain with partially reduced
affinity for starch produced a LamB protein which still permeated
maltodextrins, maltose, and non- maltose sugars but had also gained an
ability to permit the diffusion of sucrose and raffinose; in this strain
sucrose and raffinose could now compete for the starch-binding site. A
mutant with enhanced affinity for both maltose and starch produced a
protein which exhibited elevated rates of diffusion for longer
maltodextrins but still permeated other small sugars. Two other mutants
with altered affinities showed relatively minor changes in the diffusion of
maltose and non- maltose sugars. It could be concluded from these studies
that the LamB proteins form pores allowing the diffusion of a wide variety
of monosaccharides irrespective of the presence or the absence of affinity
of a binding site for maltodextrins. However, the presence of a sugar-
binding site is crucial in determining the rate of the diffusion of
maltodextrins or other oligosaccharides.
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