Receptor-independent regulation of Gα13 by alpha-1-antitrypsin C-terminal peptides

Alpha-1-antitrypsin (AAT), a circulating serine protease inhibitor, is an acute inflammatory response protein with anti-inflammatory functions. The C-terminal peptides of AAT are found in various tissues and have been proposed as putative bioactive peptides with multiple functions, but its mechanism of action remains unclear. We previously reported that a mouse AAT C-terminal peptide of 35 amino acids (mAAT-C_1-35) penetrates plasma membrane and associates guanine nucleotide-binding protein subunit alpha 13 (Gα13). Here, we show that mAAT-C_1-35 binds directly to the guanosine diphosphate (GDP)-bound form of Gα13 through the N-terminal region (mAAT-C_1-17), thereby facilitating the interaction of Gα13・GDP with its effector proteins. The minimal sequence (mAAT-C_3-16) and essential amino acid residue (Phe11) of mAAT-C_1-17 were identified as being necessary for this effect. A molecular dynamics simulation for the Gα13・GDP-mAAT-C_1-17 complex model showed that binding of mAAT-C_1-17 to the region surrounded by switch regions of Gα13 stabilizes the flexible switch II and III regions, thereby maintaining their active conformation. In addition, mAAT-C_1-35 activates the Gα13 signaling pathway in cells where Phe11 is required. Our study reveals the structure-based mechanism of action of AAT-C peptides in the regulation of Gα13 and demonstrates that AAT-C peptides represent a biological peptide capable of activating G protein signals in a manner that is independent of G-protein-coupled receptors.