Structural insights into the role of N-terminal integrity in PhoSL for core-fucosylated N-glycan recognition.

PhoSL (Pholiota squarrosa Lectin) has an exceptional binding affinity for biomolecules with core-fucosylated N-glycans. This modification involves the addition of fucose to the inner N-acetylglucosamine within the N-glycan structure and is known to influence many physiological processes. Nevertheless, the molecular interactions underlying high-affinity binding of native PhoSL to core-fucosylated N-glycans remain largely unknown. In this study, we devised a strategy to produce PhoSL with the essential structural characteristics of the native protein (n-PhoSL). To do so, a fusion protein was expressed in E. coli and purified. Then, enzymatic cleavage and incubation with glutathione were utilized to recapitulate the native primary structure and disulfide bonding pattern. Subsequently, we identified the residues crucial for n-PhoSL binding to core-fucosylated chitobiose (N2F) via NMR spectroscopy. Additionally, crystal structures were solved for both apo n-PhoSL and its N2F complex. These analyses suggested a pivotal role of the N-terminal amine in maintaining the integrity of the binding pocket and actively contributing to core-fucose recognition. In support of this idea, the inclusion of additional residues at the N-terminus considerably reduced binding affinity and PhoSL cytotoxicity toward breast cancer cells. Taken together, these findings can facilitate the utilization of PhoSL in basic research, diagnostics and therapeutic strategies.

Structural basis of transcriptional activation by the OmpR/PhoB-family response regulator PmrA.

PmrA, an OmpR/PhoB-family response regulator, triggers gene transcription responsible for polymyxin resistance in bacteria by recognizing promoters where the canonical-35 element is replaced by the pmra-box, representing the PmrA recognition sequence. Here, we report a cryo-electron microscopy (cryo-EM) structure of a bacterial PmrA-dependent transcription activation complex (TAC) containing a PmrA dimer, an RNA polymerase σ70 holoenzyme (RNAPH) and the pbgP promoter DNA. Our structure reveals that the RNAPH mainly contacts the PmrA C-terminal DNA-binding domain (DBD) via electrostatic interactions and reorients the DBD three base pairs upstream of the pmra-box, resulting in a dynamic TAC conformation. In vivo assays show that the substitution of the DNA-recognition residue eliminated its transcriptional activity, while variants with altered RNAPH-interacting residues resulted in enhanced transcriptional activity. Our findings suggest that both PmrA recognition-induced DNA distortion and PmrA promoter escape play crucial roles in its transcriptional activation.

Structural basis for -35 element recognition by σ4 chimera proteins and their interactions with PmrA response regulator.

In class II transcription activation, the transcription factor normally binds to the promoter near the -35 position and contacts the domain 4 of σ factors (σ4 ) to activate transcription. However, σ4 of σ70 appears to be poorly folded on its own. Here, by fusing σ4 with the RNA polymerase ß-flap-tip-helix, we constructed two σ4 chimera proteins, one from σ70σ470c and another from σSσ4Sc of Klebsiella pneumoniae. The two chimera proteins well folded into a monomeric form with strong binding affinities for -35 element DNA. Determining the crystal structure of σ4Sc in complex with -35 element DNA revealed that σ4Sc adopts a similar structure as σ4 in the Escherichia coli RNA polymerase σS holoenzyme and recognizes -35 element DNA specifically by several conserved residues from the helix-turn-helix motif. By using nuclear magnetic resonance (NMR), σ470c was demonstrated to recognize -35 element DNA similar to σ4Sc . Carr-Purcell-Meiboom-Gill relaxation dispersion analyses showed that the N-terminal helix and the ß-flap-tip-helix of σ470c have a concurrent transient α-helical structure and DNA binding reduced the slow dynamics on σ470c . Finally, only σ470c was shown to interact with the response regulator PmrA and its promoter DNA. The chimera proteins are capable of -35 element DNA recognition and can be used for study with transcription factors or other factors that interact with domain 4 of σ factors.