SusC/D-like proteins in Gammaproteobacteria that utilize fructans

SusC/D-like proteins are essential components of glycan utilization machineries in Bacteroidota, but remain unknown in other bacterial phyla. The glycan-binding SusD-like protein forms a lid on top of the SusC-like TonB-dependent transporter (TBDT) and both are structurally designed to function as a complex in sugar uptake. In comparison, Gammaproteobacteria import glycans using classical TBDTs without an accessory SusD-like protein. We have now identified a SusD-like protein and a SusC-like TBDT in a fructan polysaccharide utilization locus (PUL) of the marine gammaproteobacterium Pseudoalteromonas distincta, which are tandemly organized as in Bacteroidota. Proteome analysis revealed an increased production of PUL-encoded proteins during growth on inulin- and levan-type fructans. However, P. distincta preferred inulin over plant-derived levan and hardly grew on bacterial levan. Further analysis showed that the SusD-like protein has a weak affinity (Ka 43 M^-1) for oligosaccharides from inulin. The PUL-encoded glycoside hydrolase from family 32 (GH32) hydrolyzes inulin and plant-derived levan, but also has a low activity on bacterial levan, which confirms the growth experiments. Comparative genomics identified further SusC/D-like proteins in Gammaproteobacteria genomes, most of which (83%) were encoded in fructan PULs.