COLLAGEN FIBRILLOGENESIS DURING SEA-URCHIN DEVELOPMENT - RETENTION OFSURF MOTIFS FROM THE N-PROPEPTIDE OF THE 2-ALPHA CHAIN IN MATURE FIBRILS

The sea urchin 2 alpha fibrillar collagen chain has a unique amino-pro peptide structure with several repetitions of a still unknown 140-145- amino-acid, four-Cys module called SURF (for sea urchin fibrillar modu le). To follow the expression of the amino-propeptide of the 2 alpha c hain and assign a function to this domain, we have overproduced in Esc herichia coli several recombinant proteins corresponding either to the amino-propeptide or to the amino-telopeptide. Monoclonal and/or polyc lonal antibodies against these recombinant proteins allowed us to obse rve a similar tissue distribution during the first stages of developme nt A signal is first observed at the prism stage as intracellular spot s in mesenchymal cells. In plutei, immunofluorescence staining is obse rved around the skeleton spicules and as a thin meshwork surrounding t he mesenchymal cells. at the ultrastructural level, and using antibodi es against the amino-propeptide, gold particles are observed at the su rface of 25 nm thin periodic fibrils. By rotary shadowing, these fibri ls show a brush-bottle aspect, exhibiting at their surface numerous pe riodically distributed thin rods ended by a small globule. These data indicate that the amino-propeptide is maintained during fibrillogenesi s. As previously suggested, the retention of the amino-propeptide coul d play an important role in regulation of the fibril growth. We propos e that the important region of this amino-propeptide in thr widely enc ountered 25-nm-diameter fibrils is the short triple-helical segment. T he globular part of the amino-propeptide will not only restrict the fi bril growth but also interact with other neighbouring components and p laying, as suspected from our immunofluorescence studies, a function d uring the spiculogenesis of the sea urchin embryo.