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The maltose-binding protein (MalE) of Escherichia coli is the periplasmic
component of the transport system for malto-oligosaccharides. We have examined
the characteristics of a Mal- mutant of malE corresponding to the double
substitution Gly32 --> Asp/Ile33 --> Pro, MalE31, previously obtained by
random mutagenesis. In vivo, the MalE31 precursor is efficiently processed, but
the mature protein forms inclusion bodies in the periplasm. Furthermore, the
accumulation of insoluble MalE31 is independent of its cellular localization;
MalE31 lacking its signal sequence forms inclusion bodies in the cytoplasm. The
native MalE31 protein can be purified by affinity chromatography from inclusion
bodies after denaturation by 8 M urea. The renatured protein exhibits full
maltose binding affinity (Kd= 9 x 10(-7) M), suggesting that its folded
structure is similar to that of the wild-type protein. Unfolding/refolding
experiments show that MalE31 is less stable (-5. 5 kcal/mol) than the wild-type
protein (-9.5 kcal/mol) and that folding intermediates have a high tendency to
form aggregates. In conclusion, the observed phenotype of cells expressing
malE31 can be explained by a defective folding pathway of the protein.
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