Composition of spider silks. (A) Structural motifs occurring within spider silks. X indicates a residue that may vary within or between proteins. The spacer represents non-repetitive but conserved regions that disrupt the glycine-rich repeats. More details on the motifs can be found in the text. (B) The sequenced cDNAs of adf-3 and adf-4 code for the shown amino acid motifs and represent approximately 1/6th of the entire dragline silks. Both comprise a non-repetitive (NR) carboxyl-terminal region and a large repeat unit based on three major peptide motifs as visualized. The amino-terminal region is so far unresolved for any dragline silk. The predicted flagelliform silk protein is organized into non-repetitive (NR) amino-terminal and carboxyl-terminal regions that flank a repetitive region made up of 11 iterations of a repeating unit. Each unit contents approximately 440 amino acids. Three types of sub-repeats are present in an ensemble with the predominant unit being GPGXX.