Molecular Interactions of Shank Scaffolds Anchoring of Postsynaptic Membrane Proteins

Shank proteins interact directly as well as indirectly with postsynaptic membrane proteins and membrane proteins in epithelial tissues, such as kidney and lung; direct interactions are mediated by the PDZ domain, which is recognized by type I PDZ ligand motifs (sequence: .. .Xxx-Ser/Thr-Xxx-Ile/Leu-COO-) at the C-termini of various transmembrane proteins, including G-protein coupled receptors (GPCRs), cell adhesion molecules, ion channels and transport proteins (Kim et al. 2004; Kreienkamp et al. 2000; Lee et al. 2007; Olson et al. 2005; Tobaben et al. 2000; Shi et al. 2004; Schuetz et al. 2004; Uemura et al. 2004; Zhang et al. 2005; Zitzer et al. 1999a,b). Postsynaptic ionotropic glutamate receptors may also be attached directly to the PDZ domain of Shank (in the case of the GluR-52 subunit; Uemura et al. 2004). However, the majority of AMPA- and NMDA-type receptors does not bind to Shank directly; instead, these are anchored via PSD-95 proteins, also through PDZ-type interactions. In case of AMPA receptors this is achieved through the accessory protein stargazin (e.g., Chen et al. 2000), whereas NMDA receptors interact directly with PSD-95 through the C-terminal tails of their NMDAR2-subunits (Kornau et al. 1995). PSD-95 contains a domain similar to guanylate kinases, which does not harbor any enzymatic activity but binds to guanylate kinase-associated proteins (GKAP; also termed SAP-associated proteins, SAPAP1-4; Kim et al. 1997; Takeuchi et al. 1997; see Fig. 2).

GKAP/SAPAPs then interact with the PDZ domains of Shank (Boeckers et al. 1999b; Naisbitt et al. 1999; Yao et al. 1999), providing a rationale for the remote

Fig. 2 Integration of different postsynaptic glutamatergic receptor systems through Shank and the attached proteins PSD-95, SAPAP and Homer. The ability of the Shank SAM domain to multimerize is indicated

position of Shank with respect to the postsynaptic plasma membrane as described by Valtschanoff and Weinberg (2001). G-protein coupled metabotropic glutamate receptors (mGluRs) are again indirectly linked to Shank; the intermediate Homer proteins have the ability to dimerize via a coiled-coil motif (Tu et al. 1999). Through additional Ena/VASP homology (EVH) domains, Homer binds to Pro-Pro-Xxx-Xxx-Phe sequences in the mGluR C-Terminal tails and Shank, respectively, thereby physically connecting both proteins. One consequence of this complex system of interactions is a tight connection between the metabotropic and ionotropic glutamate receptor systems of the PSD (Naisbitt et al. 1999; Tu et al. 1999; see Fig. 2). However, the relevance of the Homer/Shank interaction is not limited to the functional integration of cell surface receptors, as another proline-rich ligand motif for the Homer EVH domain is found in the ER-associated receptors for inositol trisphosphate (IP3-receptors). Sala et al. (2001, 2003, 2005) have shown that through this interaction ER cisternae are targeted to dendritic spines; the ability of Shank1 to enhance the functional maturation of dendritic spines is critically dependent on its interaction with Homer and the ability to recruit ER-type membranes to the spine.

The notion that the presence of Shank is crucial for the establishment of transmitter-dependent Ca-release from intracellular stores at the postsynaptic site is further supported by the observation that an isoform of phospholipase C (PLC-03) binds to the PDZ domain of Shank2 (Hwang et al. 2005). Thus, with the exception of the heterotrimeric G-proteins, all components of mGluR/PLC-03/IP3-R/Ca-signaling in spines are organized by the Shank/Homer scaffold function (Fig. 3).

Fig. 3 Signaling complex for the activation of phospholipase C. Note that due to the dimeric nature of Homer, it could also bridge IP3 receptors and mGluR directly. Heterotrimeric G-proteins were omitted for clarity

Fig. 3 Signaling complex for the activation of phospholipase C. Note that due to the dimeric nature of Homer, it could also bridge IP3 receptors and mGluR directly. Heterotrimeric G-proteins were omitted for clarity

Interestingly, the Shank/Homer complex may also be involved in the regulation of L-type Ca-channels, another downstream effector molecule for GPCR signaling. In spiny striatal neurons, channels containing the Cav1.3 subunit bind to the PDZ and SH3 domains of Shank. Modulation of channel activity by dopaminergic (D2) and muscarinergic (M1) receptors requires association of the channel with the Shank/Homer complex, as competitive blockers of the Shank PDZ domain, as well as the Homer EVH domain, prevent this type of regulation (Olson et al. 2005; Zhang et al. 2005). At present it is unclear whether the GPCRs involved are also physically associated with Shank proteins.

0 0

Post a comment