Bd. Nelson et B. Traxler, EXPLORING THE ROLE OF INTEGRAL MEMBRANE-PROTEINS IN ATP-BINDING CASSETTE TRANSPORTERS - ANALYSIS OF A COLLECTION OF MALG INSERTION MUTANTS, Journal of bacteriology, 180(9), 1998, pp. 2507-2514
The maltose transport complex of Escherichia coli is a well-studied ex
ample of an ATP-binding cassette transporter, The complex, containing
one copy each of the integral membrane proteins MalG and MalF and two
copies of the peripheral cytoplasmic membrane protein MalK, interacts
with the periplasmic maltose-binding protein to efficiently translocat
e maltose and maltodextrins across the bacterial cytoplasmic membrane.
To investigate the role of MalG both in MalFGK(2) assembly interactio
ns and in subsequent transport interactions, we isolated and character
ized 18 different MalG mutants, each containing a 31-residue insertion
in the protein. Eight insertions mapping to distinct hydrophilic regi
ons of MalG permitted either assembly or both assembly and transport i
nteractions to occur. In particular, we isolated two insertions mappin
g to extracytoplasmic (periplasmic) regions of MalG which preserved bo
th assembly and transport abilities, suggesting that these are permiss
ive sites in the protein. Another periplasmic insertion seems to affec
t only transport-specific interactions between MalG and maltose-bindin
g protein, defining a novel class of MalG mutants. Finally, four MalG
mutant proteins, although stably expressed, are unable to assemble int
o the MalFGK(2) complex. These mutants contain insertions in only two
different hydrophilic regions of MalG, consistent with the notion that
a restricted number of domains in this protein are critical complex a
ssembly determinants. These MalG mutants will allow us to further expl
ore the intermolecular interactions of this model transporter.