Fig. 8: Model demonstrating the proposed mechanism of substrate capture and translocation by Bt1762-63.
From: Insights into SusCD-mediated glycan import by a prominent gut symbiont
Levan polysaccharide is initially bound and hydrolysed at the cell surface by the SGBP Bt1761 and GH32 endo-levanase Bt1760, respectively, although the precise role of the SGBP is unclear. A lid-open state of the transporter permits binding of transport-competent FOS. ① Contributions from both SusC and SusD to FOS binding elicits closure of the lid. ② Multiple cycles of lid opening and closing occur until the SusC “bin” is fully loaded with substrate, forming the transport-ready state of the complex. Substrate loading is communicated across the outer membrane by direct contact with the Bt1763 plug domain, inducing perturbation of the Ton box region on the periplasmic side of the plug, rendering it accessible to TonB. ③ TonB-mediated disruption/extraction of the plug permits substrate translocation. Details of the “reset” mechanism are unclear, but reinsertion of the plug is likely a prerequisite to restoring the open state of the transporter.
