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 .