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The role of the maltodextrin-binding site in determining the transport properties of the LamB protein.

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.