We found that a peptide (EP3a: TIKALVSRCRAKAAV) corresponding to the N-terniinal site of the intracellular third loop of human prostaglandin EP3 alpha receptor could activate G protein alpha-subunit directly. The activity was almost same as Mastoparan-X, a G protein activating peptide from wasp venom. The three-dimensional molecular structure of the peptide in SDS-d(25) micelles was determined by 2D H-1 NMR spectroscopy. The structure of EP3a consists of a positive charge cluster on the C-terminal helical site. The cluster was also found in several corresponding receptor peptides. Therefore, the positive charge cluster on the helical structure might play a crucial role in activation of G protein. (c) 2006 Elsevier Inc. All rights reserved.