Novel amino-terminal propeptide configuration in a fibrillar procollagen undergoing alternative splicing

We isolated overlapping cDNAs from embryonic libraries of the sea urchin Strongylocentrotus purpuratus coding for a fibrillar procollagen (2 alpha chain) with a predicted molecular mass of about 320 kDa. The deduced primary structure of the echinoid chain consists of a 265-amino acid carboxyl-propeptide, a triple helical domain made of 337 uninterrupted Gly-X-Y repeats, and an unusually long amino-propeptide. Aside from a 10-cysteine globular region, a collagenous sequence, and a nonhelical segment, this protein domain includes a novel 4-cysteine motif repeated several times. Interestingly, preliminary evidence indicates that different combinations of the 4-cysteine repeats are encoded by alternatively spliced transcripts. Irrespective of this, the sea urchin 2 alpha procollagen chain represents the longest fibrillar molecule identified to date by cDNA cloning experiments in both vertebrate and invertebrate organisms.