Identification of the carboxyl peptides of mouse procollagen IV and its implications for the assembly and structure of basement membrane procollagen.

Clusters of mouse PF-HR9 endoderm cells derived from teratocarcinoma PCC4-F cells were incubated with [3H]proline and [35S]methionine. The synthesis of pro alpha 1 IV and pro alpha 2 IV chains and their association into triple helically folded disulfide-linked molecules were followed. Short incubations and incubations with pactamycin showed that approximately 30,000 molecular weight collagenase-resistant peptides, which are destroyed by pepsin, form the carboxyl end of the pro alpha IV chains. While disulfide links bridge parts of individual peptides, the carboxyl peptides of the three chains of a molecule are not disulfide linked to each other. We propose that these peptides form the knob protrusion seen in electron micrographs of rotary shadowed procollagen IV molecules. The implications of these findings, especially for the relatively slow assembly of procollagen IV, are discussed.