Short chain collagens in sponges are encoded by a family of closely related genes

Two previously described sponge cDNAs, EmC4 and C23, respectively encoding a short chain collagen and a fibrillar collagen, were used to characterize collagen gene families in a freshwater sponge. EmC4 detected several clones when used to screen a cDNA library. Two overlapping clones, EmC13 1 and 2, were sequenced and appeared highly homologous to EmC4. Contrarily to C23, EmC4 hybridized with 10-12 fragments of genomic DNA digested with restriction endonucleases and detected 10 times more positive clones than C23 when used to screen a genomic library. The genomic clone G41 contained two closely related genes, COLNF13, corresponding to EmC13 and COLNF6. Partial characterization of COLNF13 revealed two partial exons and four complete exons of 153, 219, 207, and 144 base pairs, with split glycine codons at their boundaries. The deduced encoded protein is a short chain collagen containing two uninterrupted collagenous domains of 66 and 171 amino acids and non-collagenous domains. A characterized 207-base pair exon of COLNF6 is 77% identical with the comparable COLNF13 exon. In situ hybridization using EmC4 cDNA and electron microscopy suggested that the cells expressing these genes were secreting spongin, a non-fibrillar, surface collagen of these sponges.