Transglutaminase 2: a molecular Swiss army knife

Abstract

Transglutaminase 2 (TG2) is the most widely distributed member of the transglutaminase family with almost all cell types in the body expressing TG2 to varying extents. In addition to being widely expressed, TG2 is an extremely versatile protein exhibiting transamidating, protein disulphide isomerase and guanine and adenine nucleotide binding and hydrolyzing activities. TG2 can also act as a protein scaffold or linker. This unique protein also undergoes extreme conformational changes and exhibits localization diversity. Being mainly a cytosolic protein; it is also found in the nucleus, associated with the cell membrane (inner and outer side) and with the mitochondria, and also in the extracellular matrix. These different activities, conformations and localization need to be carefully considered while assessing the role of TG2 in physiological and pathological processes. For example, it is becoming evident that the role of TG2 in cell death processes is dependent upon the cell type, stimuli, subcellular localization and conformational state of the protein. In this review we discuss in depth the conformational and functional diversity of TG2 in the context of its role in numerous cellular processes. In particular, we have highlighted how differential localization, conformation and activities of TG2 may distinctly mediate cell death processes.

Figure 1. Reactions catalyzed by the transamidase activity of TG2

(a) TG2 can use a water molecule as an acyl-acceptor to deaminate a peptide-bound glutamine residue and convert it to a glutamate residue. (b) When a primary amine acts as acyl-acceptor, the glutamine residue is modified resulting in a posttranslational modification. (c) An isopeptide bond forms when the ε-group of a peptide bound lysine is the acyl-acceptor resulting crosslinking of two proteins. (d) The crosslink does not have to be an isopeptide bond. A primary amine can act as a crosslinker between two proteins.

Figure 2. Localization of TG2 in the cell

TG2 has no known signal sequence, and is primarily localized in the cytosolic compartment. However, a small but significant portion of TG2 is localized in the nucleus, although the mechanism(s) by which it enters the nucleus has not yet been fully defined.. Although TG2 can modify mitochondrial enzymes, the evidence suggests that it is associated with mitochondria, but not found in the mitochondria. TG2 is also found outside of the cell, both associated with the plasma membrane and deposited into the matrix.

Figure 3. Transamidase active site of TG2

The catalytic site of transamidating activity is composed of the catalytic triad: Cys277, His335 and Asp358. A conserved tryptophan residue, Trp241 is also critical for the transamidating activity. A hydrogen bond forms between Cys277 and Tyr516 in the closed conformation of TG2, which is believed to further stabilize the closed conformation and keep the enzyme inactive.

Figure 4. Guanine nucleotide binding pocket of TG2

The guanine nucleotide binding pocket is an example of a tertiary pocket, with contributing residues from the catalytic domain, β-barrel-1 and β-barrel-2 domains. Arg580 interacts with the guanine nucleotide at several points; loss of this residue significantly impairs GTP/GDP binding activity and disinhibits transamidase activity. Dashed green line depicts optimal H-bonding distance; dashed white line depicts suboptimal H-bonding distance.

Figure 5. Regulatory elements of the human TGM2 gene expression

The expression of TG2 is regulated by many factors. In this figure identified elements that regulate TG2 expression are shown. Retinoic acid response elements: RRE-1 (-1731), RRE-2 (-1720), glucocorticoid response element: GRE (-1399), nuclear factor κB response element: NF-κB (-1338), interleukin-6 response element:IL-6 (-1190), tumor growth factor-β1: TGF-β1 (-900), activator protein-2: AP-2 (-634), hypoxia response element: HRE (-367), activator protein-1: AP-1 (-183), CAAT box (-96), GC box: Sp1 binding motifs (-54, -43, +59, +65), TATA box (-29), nuclear factor-1: NF-1 (+4, +12).

Figure 6. Transcriptional regulation by TG2

(a) TG2 transamidates I-κB, leading to NF-κB activation. (b) TG2 activates NF-κB transcriptional activity. (c) TG2 increases cAMP, leading to activation of CREB. (d) TG2 downregulates MMP-9 expression and promoter activity. (e) TG2 suppresses HRE activity in response to hypoxia. (f) TG2 suppresses PGC-1α and cyt-c expression. (g) TG2 crosslinks Sp1 in response to ethanol and downregulates Sp1 regulated gene transcription.

Figure 7. Simplified hypothetical model showing different roles of TG2 in death/survival paradigm

The complex circuitry of the cellular processes regulated by TG2. Depending on its localization, conformation and activity state, TG2 can differentially modulate cell death/survival processes. Relatively well described mechanisms that are widely exploited by TG2 to control cell death/survival processes are: (i) mediating cell to ECM contacts, thereby triggering cell survival signaling; (ii) controlling the activities of transcription factors directly or indirectly to promote survival or facilitate demise.