Minst.org › Ecach › Chapter II › MalX protein

The protein discovered and designated MalX by J. Reidl and W. Boos is homologous to Enzyme IICB^glc, the two-domain glucose permease [JB].

The CRP-cAMP complex activates the transcription of malX encoding MalX as a Class II activator [FEMS Microbiology Letters] [FEMS Microbiology Letters].

Over-expression of malX was shown to allow transport of glucose (and maltose) with concomitant phosphorylation in strains lacking Enzyme IICB^glc and Enzyme IIC^man and IID^man (which are mannose-specific but can be used for glucose transport).  It was then proposed that MalX could be phosphorylated by Enzyme IIA^glc [JB].

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 IIA^glc, adenylate cyclase should be inactive (in idle mode) during glucose transport by MalX.  Indeed, mutant strains lacking Enzyme IICB^glc, Enzyme IIC^man and IID^man 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Furthermore, in such strains, Enzyme IIA^glc is required for MalX-mediated transport of glucose (M. Crasnier-Mednansky, unpublished results).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 H. Kornberg and coworkers that, in E. coli, fructose can be transported by facilitated diffusion via the glucose PTS [PNAS].