SAP102 Is a Highly Mobile MAGUK in Spines

Abstract

Membrane-associated guanylate kinases (MAGUKs), which are essential proteins in the postsynaptic density (PSD), cluster and anchor glutamate receptors and other proteins at synapses. The MAGUK family includes PSD-95, PSD-93, SAP102, and SAP97. Individual family members can compensate for one another in their ability to recruit and retain receptors at the postsynaptic membrane as shown through deletion and knock-down studies. SAP102 is highly expressed in both young and mature neurons; however, little is known about its localization and mobility at synapses. Here, we compared the distribution, mobility, and turnover times of SAP102 to the well studied MAGUK PSD-95. Using light and electron microscopy, we found that SAP102 shows a broader distribution as well as peak localization further away from the postsynaptic membrane than PSD-95. Using fluorescence recovery after photobleaching (FRAP), we found that 80% of SAP102 and 36% of PSD-95 are mobile in spines. Previous studies showed that PSD-95 was stabilized at the PSD by N-terminal palmitoylation. We found that stabilization of SAP102 at the PSD was dependent on its SH3/GK domains but not its PDZ interactions. Furthermore, we showed that stabilizing actin or blocking NMDA/AMPA receptors reduced the mobile pool of SAP102 but did not affect the mobile pool of PSD-95. Our results show significant differences in the localization, binding mechanism, and mobility of SAP102 and PSD-95. These differences and the compensatory properties of the MAGUKs point out an unrecognized versatility of the MAGUKs in their function in synaptic organization and plasticity.

The Postsynaptic Organization of Synapses

“…Measured by FRAP (fluorescence recovery after photobleaching) or by PAGFP ( photoactivatable green fluorescent protein) imaging, PSD-95 is relatively stable within the PSD, compared with other PSD components, such as SAP102, GKAP, Shank, Homer, CaMKIIa, GluA2 (GluR2), and stargazin (Kuriu et al 2006;Sturgill et al 2009;Zheng et al 2010).…”

MAGUKs: multifaceted synaptic organizers

“…Because SAP102, unlike PSD-93 and PSD-95, is not palmitoylated [Fig. 1; 27] and has been shown to move in and out of spines [28], it is an ideal protein to shuttle NMDARs in and out of the synapse.…”

Subunit-specific Regulation of N-Methyl-d-aspartate (NMDA) Receptor Trafficking by SAP102 Protein Splice Variants

“…However, how SAP102 is targeted to synapses remains unclear. Previous studies have shown that SAP102 stabilization at postsynaptic sites is dependent on the SH3 and GK domains (19). Interestingly, the I2 region of SAP102 is located within the hinge region between the SH3 and GK domains (Fig.…”

“…In contrast, SAP102 is not palmitoylated, and how SAP102 is targeted to synapses is not clear. Using protein domain truncation approaches, it has been shown that synaptic localization of SAP102 is dependent on the SH3 and GK domains (19). The different mechanisms by which PSD-95 and SAP102 are targeted to synapses suggest that they play distinct roles in receptor stabilization and/or trafficking at synapses.…”

“…The immunochemistry data were analyzed using ImageJ software (NIH). Spine/dendrite ratio was analyzed as described previously (19). Mushroom spines of secondary dendrites and adjacent dendrite shaft regions were defined under the DsRed channel.…”