Direct demonstration of the flexibility of the glycosylated proline-threonine linker in the Cellulomonas fimi Xylanase Cex through NMR spectroscopic analysis

The modular xylanase Cex (or CfXyn10A) from Cellulomonas fimi consists of an N-terminal catalytic domain and a C-terminal cellulose-binding domain, joined by a glycosylated proline-threonine (PT) linker. To characterize the conformation and dynamics of the Cex linker and the consequences of its modification, we have used NMR spectroscopy to study full-length Cex in its nonglycosylated ( approximately 47 kDa) and glycosylated ( approximately 51 kDa) forms. The PT linker lacks any predominant structure in either form as indicated by random coil amide chemical shifts. Furthermore, heteronuclear (1)H-(15)N nuclear Overhauser effect relaxation measurements demonstrate that the linker is flexible on the ns-to-ps time scale and that glycosylation partially dampens this flexibility. The catalytic and cellulose-binding domains also exhibit identical amide chemical shifts whether in isolation or in the context of either unmodified or glycosylated full-length Cex. Therefore, there are no noncovalent interactions between the two domains of Cex or between either domain and the linker. This conclusion is supported by the distinct (15)N relaxation properties of the two domains, as well as their differential alignment within a magnetic field by Pf1 phage particles. These data demonstrate that the PT linker is a flexible tether, joining the structurally independent catalytic and cellulose-binding domains of Cex in an ensemble of conformations; however, more extended forms may predominate because of restrictions imparted by the alternating proline residues. This supports the postulate that the binding-domain anchors Cex to the surface of cellulose, whereas the linker provides flexibility for the catalytic domain to hydrolyze nearby hemicellulose (xylan) chains.