Expression of collagen XVIII and localization of its glycosaminoglycan attachment sites.

Collagen XVIII is the only currently known collagen that carries heparan sulfate glycosaminoglycan side chains. The number and location of the glycosaminoglycan attachment sites in the core protein were determined by eukaryotic expression of full-length chick collagen XVIII and site-directed mutagenesis. Three Ser-Gly consensus sequences carrying glycosaminoglycan side chains were detected in the middle and N-terminal part of the core protein. One of the Ser-Gly consensus sequences carried a heparan sulfate side chain, and the remaining two had mixed chondroitin and heparan sulfate side chains; thus, recombinant collagen XVIII was a hybrid of heparan sulfate and chondroitin proteoglycan. In contrast, collagen XVIII from all chick tissues so far assayed have exclusively heparan sulfate side chains, indicating that the posttranslational modification of proteins expressed in vitro is not entirely identical to the processing that occurs in a living embryo. Incubating the various mutated collagen XVIIIs with retinal basement membranes showed that the heparan sulfate glycosaminoglycan side chains mediate the binding of collagen XVIII to basement membranes.