Different binding specificities of S-layer homology modules from Clostridium thermocellum AncA, Slp1, and Slp2.

S-layer homology (SLH) module polypeptides were derived from Clostridium thermocellum S-layer proteins Slp1 and Slp2 and cellulosome anchoring protein AncA as rSlp1-SLH, rSlp2-SLH, and rAncA-SLH respectively. Their binding specificities were investigated using C. thermocellum cell-wall preparations. rAncA-SLH associated with native peptidoglycan-containing sacculi from C. thermocellum, including both peptidoglycan and secondary cell wall polymers (SCWP), but not to hydrofluoric acid-extracted peptidoglycan-containing sacculi (HF-EPCS) lacking SCWPs, suggesting that SCWPs are responsible for binding with SLH modules of AncA. On the other hand, rSlp1-SLH and rSlp2-SLH associated with HF-EPCS, suggesting that these polypeptides had an affinity for peptidoglycan. A binding assay using a peptidoglycan fraction prepared from Escherichia coli cells definitely confirmed that rSlp1-SLH and rSlp2-SLH specifically interacted with peptidoglycan but not with SCWP.