The protein discovered and designated MalX by Reidl and Boos is homologous to Enzyme IICBGlc, the two-domain glucose permease [J Bacteriol] [MalX at UniProtKB].
The CRP-cAMP complex activates the transcription of malX encoding MalX as a Class II activator [FEMS Microbiol Lett] [FEMS Microbiol Lett].
Over-expression of malX was shown to allow transport of glucose (and maltose) with concomitant phosphorylation in strains lacking Enzyme IICBGlc and Enzyme IICMan and IIDMan (which are mannose-specific but can be used for glucose transport). It was then proposed that MalX could be phosphorylated by Enzyme IIAGlc [J Bacteriol].
When malX is expressed constitutively, growth on glucose is dependent on glucokinase. Therefore MalX can also mediate glucose transport without concomitant phosphorylation.
It can be reasoned that, if over-produced MalX is phosphorylated by Enzyme IIAGlc, adenylate cyclase should be inactive (in idle mode) during glucose transport by MalX. Indeed, mutant strains lacking Enzyme IICBGlc, Enzyme IICMan and IIDMan and glucokinase, and over-producing MalX, show a low level of cAMP when grown on glucose (as compared to other carbon sources). Furthermore, in such strains, Enzyme IIAGlc is required for MalX-mediated transport of glucose (Crasnier-Mednansky, unpublished results).
The interesting feature of the MalX protein is its capability to transport glucose with or without concomitant phosphorylation (even though with much less efficiency than the glucose PTS permease). This seems to rule out the suggestion that PTS permeases do not transport their substrates in the absence of phosphorylation. Within the same frame of thought, it was reported by Kornberg, Lambourne and Sproul that, in E. coli, fructose can be transported by facilitated diffusion via the glucose PTS [Proc Natl Acad Sci U S A].
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