Characterization of the Type‐1 Repeat from Thyroglobulin, a Cysteine‐Rich Module Found in Proteins from Different Families

The amino acid sequence of human thyroglobulin is known to enclose cysteine‐rich repetitive regions. In this study, we report the existence of an eleventh type‐1 repeat within the human thyroglobulin sequence, and we characterize the thyroglobulin type‐1 repeat as a protein module. The 11 thyroglobulin type‐1 repeats possessed the same number of cysteine residues (six in type A, four in the two type B repeats), a fairly constant number of residues between cysteines and a conserved sequence pattern. By scanning protein sequence databases, 29 proteins belonging to six different families were found to enclose at least one, and up to three, thyroglobulin type‐1 repeats in their sequence. Although the repeat was present in numerous proteins possessing binding properties, an examination of the information available in the literature showed that a direct role of the repeat in protein‐protein interaction has rarely been assessed. A distance analysis of the sequences indicated that all repeats segregate into four clusters of phylogenically close sequences. A consensus sequence of type‐1 repeats was derived from sequence similarity analysis; it comprised a central core of conserved residues including two highly conserved motifs, QC and CWCV. The type‐1 repeat from thyroglobulin was found to differ from several previously described cysteine‐rich modules, in particular from the epidermal‐growth‐factor‐like module with which it has sometimes been confused. Therefore, our results provide a complete characterization of the repeats which will help in the detection of these repeats in newly characterized proteins, a necessary step for understanding the structural/biological role of this module.