Deposition and cross-linking of newly synthesized type IV procollagen in lung matrix

Type IV procollagen (PC) is a major structural component of alveolar basement membranes; however, the mechanisms of type IV deposition in lung matrix have not been defined. We have examined the deposition and cross-linking of newly synthesized type IV in slices of adult rat lung. Type IV was rapidly deposited and selectively and covalently cross-linked in lung matrix. After a 4-h labeling with radioactive L-proline, greater than 40% of the newly synthesized type IV was insoluble in neutral salt buffers or 2 M guanidine-HCl (GuHCl). Re-extraction of the GuHCl residue in the presence of 10 mM dithiothreitol (DTT) recovered greater than 80% of the remaining type IV. By contrast, less than 20% of the total type IV PC was solubilized with 1 M NaCl containing as much as 50 mM DTT. The GuHCl plus DTT extracts contained pro alpha chains and aggregates of pro alpha chains stabilized by nondisulfide cross-links involving the pepsin-sensitive carboxy-terminal domain of the pro alpha 1 (IV) chain. There was a time-dependent increase in the recovery of type IV participating in intermolecular disulfide and nondisulfide bonds, and in the recovery of cross-linked amino-terminal domains (i.e., "7S-collagen"). Our results further support the hypothesis that the intact secreted form of type IV PC is a major structural subunit of pulmonary basement membranes, and demonstrate that the rapid insolubilization of type IV in lung matrix involves the formation of strong noncovalent interactions that are stabilized by intermolecular disulfide and nondisulfide bonds involving the terminal domains of type IV procollagen.