Figure 1.

Example of the triplets ITL, RPA, and DGR (dark-shaded letters) in the integrins of marine sponges existing in the murine (underlined) and human MHCI-CD8 complex, human collagen (DGR triplet), and human receptor heteromers: TM1, TM2 and TM7 are the first, the second and the seventh transmembrane α-helices of ACHB, CXCR4, and GABAB (GABAB1-GABAB2 heteromer) receptors, respectively, and contain the ITL triplet. The RPA triplet is also found in the TSHR and FGFR1; the RPA but not the ITL triplet homologies are in a position to contribute to the physical interaction between the beta and delta subunits of the neuromuscular nicotinic receptor (ACHB-ACHD); light-shaded letters are positively charged amino acids (R, K, and H), whereas dark-shaded white letters are negatively charged amino acids (D and E); bold letters are main players of leucine-rich motifs (L, S, and C).