Transglutaminase 2 Regulates the GTPase-activating Activity of Bcr

Abstract

Transglutaminase 2 (TG2) is a multifunctional protein that has been implicated in numerous pathologies including that of neurodegeneration and celiac disease, but the molecular interactions that mediate its diverse activities are largely unknown. Bcr and the closely related Abr negatively regulate the small G-protein Rac: loss of their combined function in vivo results in increased reactivity of innate immune cells. Bcr and Abr are GTPase-activating proteins that catalyze the hydrolysis of the GTP bound to Rac. However, how the Bcr and Abr GTPaseactivating activity is regulated is not precisely understood. We here report a novel mechanism of regulation through direct protein-protein interaction with TG2. TG2 bound to the Rac-binding pocket in the GTPase-activating domains of Bcr and Abr, blocked Bcr activity and, through this mechanism, increased levels of active GTP-bound Rac and EGF-stimulated membrane ruffling. TG2 exists in at least two different conformations. Interestingly, experiments using TG2 mutants showed that Bcr exhibits preferential binding to the non-compacted conformation of TG2, in which its catalytic domain is exposed, but transamidation is not needed for the interaction. Thus, TG2 regulates levels of cellular GTP-bound Rac and actin cytoskeletal reorganization through a new mechanism involving direct inhibition of Bcr GTPase-activating activity.

Metastasis-associated S100A4 is a specific amine donor and an activity-independent binding partner of transglutaminase-2

“…It is not unprecedented that TG2 interacts with partner proteins in the open form without being a substrate of TG2; Bcr, a guanine nucleotide activating protein was also shown to bind to the open conformation of TG2 [21].…”

“…An inactive TG2 mutant (that has still preserved its guanine nucleotide binding ability) is a useful tool in studying TG2 interaction partners and substrates. One extensively used TG2 mutant is Cys277Ser where the cysteine residue that is part of the catalytic triad is mutated to serine [21,22]. However, this mutation results in a conformational change that impairs GTP binding capability [23].…”

Physiological, pathological, and structural implications of non-enzymatic protein–protein interactions of the multifunctional human transglutaminase 2

“…Growth factor receptors and low-density lipoprotein receptors (LDL) as TG2-interacting partners Extracellular TG2 was shown to interact with two growth factor receptors, platelet-derived growth factor receptor (PDGFR) and vascular endothelial growth factor receptor [82] Physiological, pathological and structural… [150] Physiological, pathological and structural… Interacting partners of TG2 collected from a literature search have been assembled in this table. The type of cell line and cellular locations, sequence of interaction, methodology used, and their proposed functions have been provided for each interacting partner.…”

“…The Rho family of small GTPases, including Rho, Rac, and Cdc42, are critical regulators of a variety of cellular functions including cytoskeleton rearrangement and membrane trafficking, which are essential during cell adhesion and migration processes [82]. GTPase-activating proteins (GAPs), such as Bcr and Abr, regulate the GTPase activity of these proteins.…”

“…TG2 was found to interact directly with the C-terminal GAP domain of Bcr and Abr using COS-1 and Swiss 3T3 cells. The interaction was more prominent when TG2 was in an open confirmation because the addition of calcium or GTP binding mutant forms of TG2 both increased the level of the TG2-Bcr/Abr interaction [82]. Bcr was later reported to serve as a substrate of TG2 in human pulmonary artery endothelial cell lines (HPAECs) but crosslinking was only observed under specific and extreme stress conditions.…”