How Do the G-Protein-Linked Receptor and an Ion Channel Differ From Each Other?
Just as a journey of a thousand miles begins with a single step, so a complex signaling pathway inside of a cell begins with a single key event – the binding of a signaling molecule, or ligand, to its receiving molecule, or receptor. Receptors and ligands come in many forms, but they all have one thing in common: they come in closely matched pairs, with a receptor recognizing just one (or a few) specific ligands, and a ligand binding to just one (or a few) target receptors. Binding of a ligand to a receptor changes its shape or activity, allowing it to transmit a signal or directly produce a change inside of the cell.
Intracellular signaling, the cellular tool for communication, is a very complex and diverse process. This process relies on elaborate systems of proteins which enable cells to respond to a variety of stimuli in their internal or external environment. Among these membrane proteins is the GPCRs Superfamily. They form the largest and most diverse family of cell surface receptors and proteins. For example, there are about 1000 GPCRs in neurons alone. The diversity of this superfamily is a result of the large number of members comprising this family, their ability to form different dimer combinations and their ability to respond to a multitude of stimuli, as well as by the large number of intracellular signaling pathways they activate. Despite their structural and functional diversity, all GPCRs share a similar molecular architecture. They consist of seven transmembrane domains, linked by alternating intracellular and extracellular loops, an extracellular N-terminus and an intracellular C-terminus.
Ion channel receptors are a vital component of nervous system signaling, allowing rapid and direct conversion of a chemical neurotransmitter message to an electrical current (Jarvis S. E., Magga J. M, 2000). In recent decades, it has become apparent that ionotropic receptors are regulated by protein-protein interactions with other ion channels, G-protein coupled receptors and intracellular proteins. These other proteins can also be modulated by these interactions with ion channel receptors. This bidirectional functional cross-talk is important for critical cellular functions such as excitotoxicity in pathological and disease states like stroke, and for the basic dynamics of activity-dependent synaptic plasticity. Protein interactions with ion channel receptors can therefore increase the computational capacity of neuronal signaling cascades and also represent a novel target for therapeutic intervention in neuropsychiatric disease (Kalia L. V., Pitcher G. M). This review will highlight some examples of ion channel receptor interactions and their potential clinical utility for neuroprotection.
To conclude, the comparative slowness of metabotropic receptor actions reflects the fact that multiple proteins need to bind to each other sequentially in order to produce the final physiological response. Importantly, a given transmitter may activate both metabotropic receptors and ligand-gated ion channels to produce both fast and slow PSPs at the same synapse. Perhaps the most important principle to keep in mind is that the response elicited by a given neurotransmitter is determined by the postsynaptic complement of receptors and their associated channels. Exactly how postsynaptic responses are produced by some especially important examples of neurotransmitter receptors is considered in the following sections.
Hollmann M., Boulter J., Maron C., Beasley L., Sullivan J., Pecht G., et al. (1993). Zinc potentiates agonist-induced currents at certain splice variants of the NMDA receptor. Neuron 10
Jarvis S. E., Magga J. M., Beedle A. M., Braun J. E., Zamponi G. W. (2000). G protein modulation of N-type calcium channels is facilitated by physical interactions between syntaxin 1A and Gbetagamma. J. Biol. Chem. 275
Kalia L. V., Pitcher G. M., Pelkey K. A., Salter M. W. (2006). PSD-95 is a negative regulator of the tyrosine kinase Src in the NMDA receptor complex. EMBO J. 25, 4971–4982