Protein modifications involved in neurotransmitter and gasotransmitter signaling

Abstract

Covalent modifications of intracellular proteins, such as phosphorylation, are generally thought to occur as secondary or tertiary responses to neurotransmitters, following the intermediation of membrane receptors and second messengers such as cyclic AMP. By contrast, the gasotransmitter nitric oxide directly S-nitrosylates cysteine residues in diverse intracellular proteins. Recently, hydrogen sulfide has been acknowledged as a gasotransmitter, which analogously sulfhydrates cysteine residues in proteins. Cysteine residues are also modified by palmitoylation in response to neurotransmitter signaling, possibly in reciprocity with S-nitrosylation. Neurotransmission also elicits sumoylation and acetylation of lysine residues within diverse proteins. This review addresses how these recently appreciated protein modifications impact our thinking about ways in which neurotransmission regulates intracellular protein disposition.

Figure 1

An apoptotic cell death cascade involving S-nitrosylation (S-NO) of GAPDH (glyceraldehyde-3-phosphate dehydrogenase), its nuclear translocation and acetylation of nuclear targets. Diverse apoptotic stimuli, including the pathological overactivation of NMDARs, can lead to the activation of nNOS. The subsequent generation of NO results in the S-NO of GAPDH, which abolishes its catalytic activity and confers on it the ability to bind to Siah1 (seven in absentia homolog 1), an E3-ubiquitin-ligase with a nuclear localization signal (NLS). Siah1, a protein with a rapid turnover rate, is degraded by the ubiquitin proteasome system (represented by Siah with dashed line in between two parts) so that under basal conditions tissue levels are extremely low. GAPDH stabilizes the rapidly turning over Siah1 (oval shaped without dashed line), augmenting its nuclear levels. The GAPDH-Siah1 protein complex, in turn, translocates to the nucleus, where GAPDH is acetylated at lysine residue 160 by the acetyltransferase p300/CREB binding protein (CBP) through direct protein interaction, which, in turn, stimulates the acetylation and catalytic activity of p300/CBP. Consequently, downstream targets of p300/CBP, such as p53, are activated and cause neuronal cell death through the transcriptional activation of apoptotic-inducing genes such as PUMA (p53 upregulated modulator of apoptosis). PUMA is a pro-apoptotic member of the Bcl-2 protein family which mediates apoptosis associated with p53 activity. GOSPEL (GAPDH’s competitor Of Siah Protein Enhances Life), physiologically binds GAPDH, in competition with Siah, retaining GAPDH in the cytosol and preventing its nuclear translocation. S-nitrosylation of GOSPEL enhances GAPDH–GOSPEL binding and the neuroprotective actions of GOSPEL.

Figure 2

The SUMO modification cycle. SUMO precursor proteins contain a C-terminal extension which is cleaved by SUMO-specific carboxyl-terminal hydrolase activity of a sentrin-specific protease (SENP) enzyme to produce a carboxyl-terminal diglycine motif in a process called ‘SUMO maturation’. The mature SUMO is activated by an E1 SUMO activating enzyme (SAE1/2) which utilizes ATP and links the C-terminal glycine of SUMO by a thioester bond to a cysteine in the E1 activating enzyme. During the conjugation step, the SUMO moiety is transferred from the E1 activating enzyme to Ubc9, the SUMO E2 enzyme, which then attaches the SUMO to a lysine residue in the target protein that is typically, but not always, found within the consensus sequence ΨKxE (Ψ represents hydrophobic amino acids). SUMO E3 proteins stimulate protein sumoylation by associating with both UBC9 and substrates to increase the efficiency of the modification reaction by a process called target recognition and ligation. The SUMO E3 proteins identified to date include members of the family of Protein Inhibitor of Activated STAT (PIAS) family of proteins (PIAS1, PIAS3, PIASx and PIASy), the polycomb protein 2 (Pc2), and Ran-binding protein 2 (RANBP2). SUMO groups are removed from proteins by SENPs, of which there are six human forms. Rhes, a small G-protein, is selectively localized to the corpus striatum and binds directly to both E1 and Ubc9, enhancing cross-sumoylation as well as thioester transfer from E1 to Ubc9.