Analyses of Mlc–IIB ^Glc interaction and a plausible molecular mechanism of Mlc inactivation by membrane sequestration

Abstract

In Escherichia coli, glucose-dependent transcriptional induction of genes encoding a variety of sugar-metabolizing enzymes and transport systems is mediated by the phosphorylation state-dependent interaction of membrane-bound enzyme IICB Glc (EIICB Glc ) with the global repressor Mlc. Here we report the crystal structure of a tetrameric Mlc in a complex with four molecules of enzyme IIB Glc (EIIB), the cytoplasmic domain of EIICB Glc . Each monomer of Mlc has one bound EIIB molecule, indicating the 1:1 stoichiometry. The detailed view of the interface, along with the high-resolution structure of EIIB containing a sulfate ion at the phosphorylation site, suggests that the phosphorylation-induced steric hindrance and disturbance of polar intermolecular interactions impede complex formation. Furthermore, we reveal that Mlc possesses a built-in flexibility for the structural adaptation to its target DNA and that interaction of Mlc with EIIB fused only to dimeric proteins resulted in the loss of its DNA binding ability, suggesting that flexibility of the Mlc structure is indispensable for its DNA binding.enzyme IICB Glc ͉ glucose signaling ͉ protein-protein interaction ͉ transcription regulation B acteria sense continuous changes in their environment and adapt metabolically to compete effectively with other organisms for limiting nutrients. One sensory transduction system monitoring availability of a certain group of carbon sources is the phosphoenolpyruvate:sugar phosphotransferase system (PTS) (1). In addition to concomitant transport and phosphorylation of sugars, PTSs take part in a variety of physiological processes through direct interactions with their target proteins (2-5).It is a normal occurrence that cells make more proteins necessary to transport and metabolize PTS sugars when they encounter an environment where PTS sugars are available. Glucose, a representative PTS sugar, mediates transcriptional activation of several genes for PTS-related sugar transporters and some enzymes involved in glycolysis (1, 6). Mlc, a tetrameric protein (7), is a signaling mediator for glucose induction of several PTS operons and related genes (8)(9)(10)(11)(12)(13)(14).A unique feature of induction of the Mlc regulon by glucose is that the effector molecule modulating Mlc activity is a membranebound protein, EIICB Glc (7,15,16), in which cytosolic EIIB protein is attached to membrane-embedded EIIC protein through a linker of Ͼ20 aa. In the absence of glucose, EIICB Glc mainly exists in the phosphorylated form (pEIICB Glc ). Because Mlc cannot interact with pEIICB Glc , Mlc dissociates from pEIICB Glc to bind to target promoters and repress their transcription. When glucose is available, transported glucose takes the phospho-groups away from pEIICB Glc and the dephosphorylated EIICB Glc recruits Mlc to sequester it from its target promoters. This leads to increased synthesis of EIICB Glc and other PTS proteins necessary to take up and metabolize glucose more efficiently.The physiological importance of the glucose-specific enz...

Structure-Function Studies of the Staphylococcal Methicillin Resistance Antirepressor MecR2

“…MecR2 NDD likewise conforms to the structural determinants of such a DBD (see above). The reported structures of Mlc, T. maritima, and V. cholerae are DNA-unbound, and they display the two recognition helices of a dimer in a relative spatial arrangement that is not adequate for binding to two successive turns of the major groove of dsDNA (60,67). This is consistent with the finding that structural flexibility-which allows for major structural rearrangement-of Mlc was identified as essential for DNA binding activity and regulatory function (67).…”

“…XylR still binds and is allosterically regulated by sugar. Mlc would represent the next step-as already anticipated in (60)-to a three-domain DNA-binding transcriptional repressor that does not bind sugar and is not regulated by binding to an inducer or by proteolytic cleavage but through sequestration by a glucose transporter, i.e., through a protein-protein interaction (67,76). Finally, MecR2 would represent a last step in the evolutionary process, in which a three-domain ligand-independent Mlc-like repressor would have kept an unspecific DNA binding ability putatively required for biological function as an antirepressor.…”

“…However, there is no structural data on XylR available. E. coli protein Mlc is the only functionally and structurally characterized ROK family protein with DNA repressor function (60,67). Mlc is a dimeric/tetrameric transcriptional repressor that controls the utilization of glucose in E. coli (68).…”

The Bacterial Phosphoenolpyruvate:Carbohydrate Phosphotransferase System: Regulation by Protein Phosphorylation and Phosphorylation-Dependent Protein-Protein Interactions

“…In fact, EIIB Glc binds to Mlc in the central part of the protein between the N-terminal DNA binding motif and the C-terminal dimerization domain. This interaction lowers the affinity of the repressor for its target sites (142). However, interaction with the EIIB Glc domain alone does not seem to be sufficient for Mlc inactivation.…”

“…Possibly, cytoplasmic EIIB Glc has a lower affinity for Mlc and might therefore have lost its inhibiting effect. However, fusing the EIIB Glc domain to another membrane protein prevented Mlc activity, suggesting that the interaction with the membrane environment is important for Mlc regulation (141,142). The crystal structure of the complex formed by the EIIB Glc domain and Mlc revealed that Mlc forms tetramers with each subunit binding an EIIB Glc domain.…”