Characterization of Underwater Silk Proteins from Caddisfly Larva, Stenopsyche marmorata

The underwater silk threads of a caddisfly larva, Stenopsyche marmorata, is composed of four kinds of proteins, which are designated as Smsp-1 to -4. Smsp-1 has been purified in a electrophoretic homogeneity, and the peptide sequencing of the Smsp-1 tryptic digests indicated the presence of eight kinds of characteristic segments, most of which are enriched in Pro residues. The segments 3 and 4 are the 14-residue sequences of SLGPYGDXLGPYGG (X = G, D, or V) and GVGPYGDGLGPYGG, respectively. Partial molecular cloning of the Smsp-1-C-end region suggested that the hydrophobic segment 3,4-repeating moieties occupy mostly a half of the entire primary sequence of Smsp-1 gene. The segments 7 and 8 contain highly phosphorylated regions, where the Ser residues are exclusively modified in the cluster-like sequences as –(Xaa S) n – (Xaa = G, A, V, or I; S = O-phospho-L-serine; n = 3 or 4). The O-phosphorylation of Smsp-1 may switch the protein function as the fiber-forming components to the underwater cements for building the pupal case in the late 5th instar stage of the larvae.