Long-range periodic sequence of the cement/silk protein of <i>Stenopsyche marmorata</i>: purification and biochemical characterisation

The long-range periodic amino acid sequence of the bifunctional silk/cement protein from larvae of the caddisfly, <i>Stenopsyche marmorata</i>, is discussed in this study. The protein, named the <i>S. marmorata</i> silk protein (Smsp-1), was first purified to electrophoretic homogeneity. The results of Edman-based sequencing of Smsp-1 tryptic digests were consistent with the amino acid sequence deduced from a cDNA clone of the Smsp-1 gene. All undetected amino acids in the Edman-based sequencing were encoded as Ser, suggesting the presence of <i>O</i>-phospho-Ser. ³¹P-NMR and an <i>O</i>-phospho-amino acid analysis successfully showed that the <i>O</i>-phospho-Ser residue occurred in a clustered manner, serving a cement function for Smsp-1. Two patterns of non-phosphorylated repeats, –SLGPYGDPRGDXLGPYGG– (X = V, G or D) and –GVGPYGDGLGPYGG–, were enriched in Smsp-1 compared with the <i>O</i>-phospho-Ser cluster, and have fibre-forming functions.