Glutamate receptor-interacting protein (GRIP) refers to either a family of proteins that bind to the glutamate receptor or specifically to the GRIP1 protein within this family. Proteins in the glutamate receptor-interacting protein (GRIP) family have been shown to interact with GluR2, a common subunit in the AMPA receptor. This subunit also interacts with other proteins such as protein interacting with C-kinase1 (PICK1) and N-ethylmaleimide-sensitive fusion protein (NSF). Studies have begun to elucidate its function; however, much is still to be learned about these proteins.
Discovery and history of GRIP 1
The discovery of the Glutamate Receptor Interacting Protein (GRIP-1) came as a result of the observation that Glutamate Receptors, such as the NMDA receptor, cluster at synapses. Shortly after this observation, researchers identified a region on the C-terminal region of NMDA receptors called the tSXV motif that has the ability to bind to the PDZ domain of the PSD-95 protein.
Research on NMDA receptor localization paved the way for research on non-NMDA receptors such as AMPA receptors. Similar to NMDA receptors, it was discovered that AMPA receptors localize in the synaptic terminal of neurons in the central nervous system. By using GFP (green fluorescent protein) antibodies that correspond to the GRIP protein, researchers were able to use fluorescence to determine the location of GRIP in hippocampal neurons. Another GFP antibody was then used to label the GluR2 subunit of AMPA receptors. By using immunocytochemistry and comparing the location of GRIP and AMPA receptors it was determined that GRIP and AMPA receptors experience colocalization in hippocampal neurons. These findings confirmed the initial hypothesis that the GRIP protein plays an important role in binding AMPA receptors to excitatory synapses.
The structure of GRIP contains seven PDZ domains and binds to the C-terminus of the GluR2 subunit of AMPA receptors. Although the number of PDZ domains is different for the proteins PSD-95 and GRIP, the PDZ domain is a common structural motif in proteins that help mediate protein-protein interactions. The AMPA receptor amino acid sequence that the GRIP protein binds to is ESVKI. The conserved serine amino acid in the C- terminus of both AMPA and NMDA receptors suggests that it plays an important role in facilitating the interaction for GRIP and PSD-95.
Role of GRIP in AMPAR cycling
AMPA receptors are constantly being transported between the cell membrane and intracellular space and it was originally thought that GRIP may be responsible for the clustering of AMPA receptors at the excitatory synapse. Although it is still unclear the exact role of GRIP in this trafficking, It appears that PICK1 is more directly responsible for the clustering of AMPA receptors at the surface and that GRIP is involved in the stabilization of AMPA receptors intracellularly. One study showed that when the interaction between GluR2 and GRIP is disrupted, there are no changes in the surface expression of AMPA receptors or the constitutive internalization of AMPA receptors. There is, however, a reduced amount of receptors that remain internalized when receptor cycling is modified by application of AMPA-1. The ratio returns to normal when constitutive recycling is allowed to happen, suggesting that the stabilization of intracellular receptors is critical only under AMPA-induced internalization.
In later studies, two proteins, GRIP-1 (often reduced to GRIP) and ABP-L (also named GRIP-2), were found to be expressed by two separate genes and their respective contributions to AMPA receptor cycling have since been well studied. Each of these proteins have different isoforms due to differential RNA splicing. The isoforms of GRIP-1 are named GRIP-1a and GRIP-1b while those of ABP-L are distinguished as ABP-L and pABP-L. The apparent difference in both cases is that one isoform (GRIP1b and pABP-L respectively) is capable of being conjugated with Palmitic acid, an action called Palmitoylation.
Whereas GRIP initially was thought to be involved in the stabilization of AMPA receptors either at the cell surface or intracellularly when internalization was triggered by AMPA stimulation, it now appears that the GRIP-1 isoforms are involved differentially with the stabilization of AMPA receptors after being internalized due to NMDA stimulation. GRIP-1a has been shown to reduce the expected intracellular levels of AMPA receptors after NMDA stimulation. Conversely, GRIP-1b increases intracellular levels of AMPA receptors under the same conditions.