The best known family B, or Type II, G-protein-coupled receptors (GPCRs) recognize peptides as ligands. The receptors for corticotrophin-releasing factor, parathyroid hormone and secretin typify this group. However, there are only 15 such GPCRs. Many other receptors share sequence homology and have been assigned to this family. The ten ‘Frizzled’ and one ‘Smoothened’ receptors show the lowest sequence homology and are not necessarily G-protein coupled. Drosophila genetics have enabled our understanding of their biology. In contrast, relatively little is known about the largest group with family B, the 33 ‘large amino termini’ or large N-terminal family B seven-transmembrane (LNB 7TM) receptors. This review highlights the similarities found between family B receptors and provides a classification of LNB 7TM receptors.
A Putative Selectivity Filter in the G-protein-coupled Receptors for Parathyroid Hormone and Secretin