The alpha1(VIII) and alpha2(VIII) chains of type VIII collagen can form stable homotrimeric molecules

Type VIII collagen is a short chain collagen. Two chains have been described, alpha1(VIII) and alpha2(VIII), but the chain composition of type VIII collagen is far from resolved. To address this question, we have expressed full-length alpha1(VIII) and alpha2(VIII) chains in an in vitro translation system supplemented with semipermeabilized cells. Both chains gave a translation product of approximately 80 kDa that could be shown to produce a chymotrypsin/trypsin-resistant product of approximately 60 kDa, indicating that both chains could form homotrimers. Hydroxylation of proline residues was a prerequisite for stable trimer formation. The melting temperature for the alpha1(VIII) homotrimer was 45 degreesC, whereas that for alpha2(VIII) was 42 degreesC. The ability of both chains of type VIII collagen to form stable triple helices suggests that there may be different forms of this collagen and that cells may modulate the chain composition in response to different biological conditions.