Serotonylation
From Wikipedia, the free encyclopedia
| Part of | Cell, Extracellular Matrix |
| Located | Nucleus, Cytoplasm |
| Category | Post-translational Modification |
| 2003 | Diego Walther & Colleagues Discover protein serotonylation in small GTPases for the first time |
| 2019 | Lorna Farrelly & Colleagues Discover histone serotonylation as a novel post-translational modification |
| 2021 | Di Yi & Colleagues Uncover link between protein serotonylation and cancer for the first time |
Protein serotonylation refers to the post-translational modification in which the monoamine serotonin is covalently attached to glutamine residues on substrate proteins via transamidation. Serotonylation is a type of monoaminylation, which itself refers to the overall class of post-translational modifications involving monoamines. However, monoaminylation reactions are further classified by the individual monoamine reactant they describe (ie., serotonylation, dopaminylation, histaminylation).
Serotonylation has been reported for both histone and non-histone protein substrates, and thus represents a distinct neuroepigenetic and neuroproteomic regulatory mechanism with various implications in health and disease.[1] Since 2003, multiple studies have revealed the critical role of serotonylation in mediating a wide range of physiological processes, both in the nervous system and beyond.[1][2][3] Serotonylation is known to contribute to several significant diseases, including neuropsychiatric disorders such as depression and schizophrenia, as well as a variety of cancers.[1][4][5]
To date, notable protein serotonylation substrates include several metabolic enzymes (GAPDH, mTOR),[6][7] Rab GTPases (Rab3a, Rab27a),[8][9] Rho GTPases (RhoA, Rac1, Cdc42),[10][11][12] proteins involved in muscle contractility (⍺-actinin, SERCA2a),[13][14] extracellular matrix proteins (fibronectin),[15][16] neural surface proteins,[17] and Ras,[18] as well as the histone protein H3.[19][20][21]
Serotonylation has been reported in various cell types and tissues, including both serotonergic and dopaminergic neurons,[22][23] enterochromaffin cells,[24] cancer-associated fibroblasts,[25] pancreatic 𝛽-cells,[26] CD8+ T cells,[27] pulmonary endothelial cells,[28] platelets,[29] neutrophils,[21] mammary epithelial cells,[30] vascular smooth muscle,[31] and cells of the intestines.[18] Serotonylation is known to influence both tumorigenesis and cancer metastasis, and has been implicated in several types of cancer, including colorectal cancer,[7][32] neuroendocrine prostate cancer,[33] pancreatic cancer,[34] hepatocellular carcinoma,[35][36] and ependymomas (brain cancer).[37]
Protein monoaminylation was first discovered in 1957 by Heinrich Waelsch and colleagues at Columbia University. After demonstrating the incorporation of monoamines into proteins via transamidation at glutamine residues,[38] the group went on to uncover the enzyme catalyzing these reactions, effectively naming it "transglutaminase" after its function.[39][40]
Despite its discovery in the mid-twentieth century, protein monoaminylation was not investigated as a post-translational modification until 2003, when Diego Walther and colleagues at the Max-Planck-Institute for Molecular Genetics revealed that serotonylation of small GTPases mediates ⍺-granule release during the activation and aggregation of platelets.[29]
Notably, monoaminylation itself was not uncovered as an epigenetic regulatory mechanism until 2019, when Lorna Farrelly and colleagues at the Icahn School of Medicine reported the H3Q5-serotonylation (H3Q5ser) modification for the first time.[41] Thereafter, in 2021, Di Ye and colleagues at Sichuan University revealed mTOR-serotonylation as part of a novel feedback mechanism within the tryptophan pathway in colon cancer, demonstrating a link between protein serotonylation and cancer for the first time.[7]
Mechanism
Serotonylation is catalyzed by transglutaminase 2 (TGM2) in a calcium-dependent manner, and relies upon the intracellular bioavailability of serotonin substrates.[2][42] Generally, protein serotonylation occurs in the cytoplasm; however, histone serotonylation only occurs within the nucleus.[1][2] Nevertheless, the mechanism for TGM2-catalyzed serotonylation is identical for both histone and non-histone proteins alike.[1]
Structurally, Ca2+ binds directly to TGM2 itself and not to the substrate molecule.[42] Once Ca2+ binds to TGM2, a 4 nm relaxation about the major axis of the protein exposes the active site to available substrates.[42][43] The active site itself is composed of a well conserved catalytic triad (Cys277–His335–Asp358) situated within a substrate binding channel, which is bordered by two conserved residues (Trp241 and Trp332) that facilitate catalysis through stabilization of the transition state.[42][44] Once intracellular Ca2+ binds to TGM2 and exposes the substrate binding channel, the glutamine residue of a substrate protein (ie., histone H3, RhoA) is free to enter the enzyme active site.[1][42] As a transamidation reaction, the mechanism for protein serotonylation can be summarized in two parts: an initial thioester formation, followed by isopeptide bond formation.
Fig. 1 Mechanism for Protein Serotonylation
Serotonylation is a two step, Ca2+-dependent reaction in which TGM2 catalyzes the covalent attachment of a serotonin molecule onto the glutamine residue of a substrate protein. (A) The catalytic cysteine residue (Cys277) of TGM2 facilitates an initial acyl transfer reaction, which is ultimately followed by isopeptide bond formation (B). Common substrate proteins include Histone H3, small GTPases (RhoA, Rab3a), and extracellular matrix proteins (fibronectin).
When intracellular Ca2+ and serotonin concentrations are sufficient, TGM2-catalyzed serotonylation of substrate proteins can occur.[42] First, the catalytic cysteine residue (Cys277) in the TGM2 active site nucleophilically attacks the 𝛾-carboxamido group of the glutamine residue in an acyl transfer reaction (Fig. 1A), forming a thioester intermediate and releasing one molecule of ammonia (NH3) as a result.[1][42] Next, the deprotonated primary amine of the serotonin molecule nucleophilically attacks the 𝛾-thioester group of the intermediate, forming a stable isopeptide bond and ultimately releasing the enzyme (Fig. 1B).[1][42]