Conclusion
We have shown that an unstructured and flexible linker of 10 residues (Flexible, consisting of the glycine- and serine-rich sequence, N-SGGGGSGGGG-C) resists degradation by proteases present in trace amounts in solution, and also to a significant extent by exogenously added Subtilisin A, when it is flanked by two well-folded domains that are themselves also resistant to proteolysis. In addition, we have presented evidence indicating that the resistance of Flexible to proteolysis owes to its facilitation of motions by its flanking domains which presumably sterically inhibit the access of proteases to the linker. This argument suitably explains why Rigid undergoes proteolytic degradation, but not why the naturally-occurring linker,Nat-full , displays high proteolytic susceptibility. We suggest that linkers in CipA (from which Nat-full is derived) are rich in proline and threonine residues that tend to be heavily glycosylated (Gerwig et al ., 1993), since linkers joining domains in bacterial cellulases are known to be protected from proteolysis by glycosylation (Langsford et el., 1987), which does not occur during production inE. coli .