Ion Channel Clustering by Membrane-associated Guanylate Kinases

Abstract

Neurotransmission requires appropriate assembly of signal transduction machinery at synaptic sites. Although mechanisms for organization of these synaptic signaling complexes are unclear, recent studies suggest a general role for PDZ domain-containing membrane-associated guanylate kinases (MAGUK) 1 in receptor clustering at pre-and postsynaptic sites (1-4).Molecular cloning has identified four neuronal MAGUK proteins in mammals: PSD-95 (SAP-90), PSD-93 (Chapsyn-110), SAP-97 (hDLG), and SAP-102 (5-10). Immunohistochemical studies indicate that PSD-95 and PSD-93 occur primarily in a somatodendritic distribution and are specifically enriched at the PSD (11). Consistent with this postsynaptic localization, targeted disruption of PSD-95 in mouse disrupts N-methyl-Daspartate receptor-dependent synaptic plasticity (12). Recent studies show that SAP-102 can be found at postsynaptic sites (7, 13). On the other hand, SAP-97 occurs in presynaptic terminals in forebrain neurons (8). Although SAP-97 knockout mice have not yet been reported, SAP-97 is the mammalian homologue of Drosophila discs large (DLG), and flies lacking DLG show prominent presynaptic defects (14).Mechanisms for differential localization and function of MAGUK proteins remain uncertain. MAGUK proteins share extensive sequence homology in their PDZ, SH3, and guanylate kinase domains. Significant differences between these proteins occur primarily at their N termini, suggesting roles for these regions in differential functions of these proteins. Importantly, PSD-95 contains a pair of cysteine residues that are sites for protein palmitoylation (15), a post-translational modification that is essential for postsynaptic sorting (16). In contrast, SAP-97 lacks N-terminal cysteines, and it is not palmitoylated. PSD-93 is alternatively spliced at the N terminus to yield two major isoforms: the first, which we now term PSD-93␣, has cysteines at positions 5, 7, and 12; and the second, which we term PSD-93␤, has cysteines at positions 3 and 5, like PSD-95 (9). Finally SAP-102 has cysteines at positions 7, 8, 10, and 13 (7). It is uncertain whether PSD-93 or SAP-102 are palmitoylated and whether this regulates cellular trafficking and ion channel clustering by these proteins.Here, we report that both PSD-95 and PSD-93 occur prominently at postsynaptic sites in hippocampal neurons, whereas SAP-102 localizes to both dendrites and axons in these cells. Similarly, we find that PSD-95 and both isoforms of PSD-93 mediate surface ion channel clustering, although SAP-97 and SAP-102 do not. Metabolic labeling studies show that PSD-95 and both isoforms of PSD-93 are robustly palmitoylated and that SAP-97 and SAP-102 are not. Mutants of PSD-95 and PSD-93 that disrupt palmitoylation also block surface receptor clustering, indicating a critical role for this lipid modification in ion channel aggregation at the plasma membrane. Rather than being palmitoylated, we find that N-terminal cysteines in SAP-102 tightly bind to zinc and that this unique N terminus of SAP-102 confers axonal t...

Developmental loss of miniature N -methyl- d -aspartate receptor currents in NR2A knockout mice

“…The mechanism of such a displacement is unknown. Because PSD-95 is palmitoylated, whereas SAP102 is not (39)(40)(41), perhaps PSD-95 is capable of associating with plasma membranes without binding to transmembrane receptors and may displace the SAP102 system as a result of this lipid association.…”

An Alternatively Spliced Isoform of PSD-93/Chapsyn 110 Binds to the Inwardly Rectifying Potassium Channel, Kir2.1

“…This ability to interact with different MAGUKs may reflect a fundamental requirement for differential targeting of Kir2 channels within cells. PSD-95 and PSD-93, for example, are found predominantly at postsynaptic sites within neurons, whereas SAP102 is also found in axons and some presynaptic terminals (22). SAP-97 localizes to presynaptic terminals and axons and is unusual among the MAGUKs in that it is less tightly associated with the membrane (23).…”

“…SAP-97 localizes to presynaptic terminals and axons and is unusual among the MAGUKs in that it is less tightly associated with the membrane (23). Additionally, PSD-95 and PSD-93 mediate surface ion channel and receptor clustering, whereas SAP-102 and SAP-97 do not (13,22). It is believed that differences in subcellular localization and surface distribution arise from diverse targeting signals located in the N-terminal regions of MAGUK proteins (22, 24 -26).…”

A balance between excitatory and inhibitory synapses is controlled by PSD-95 and neuroligin

“…Moreover, PSD-95 expression correlates with the period of excitatory synapse maturation (7,(9)(10)(11). Augmentation of excitatory synapse activity and ion channel clustering is driven by PSD-95 but not by related proteins, including synapse-associated protein (SAP)-102 and SAP-97 (12)(13)(14), and PSD-95 regulates clustering and activity of the ␣-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid-type glutamate receptors through a direct interaction with stargazin (7,(13)(14)(15)(16)(17)(18)(19)(20). However, it is unknown how PSD-95 effects are translated into changes in apposing presynaptic terminals.…”