Transport Adaptors At Postsynaptic Scaffold Formations

With focus on selective transport, components that connect motors and cargoes are of particular interest. Transmembrane cargoes, which are transported as integral membrane proteins of transport vesicles, connect to molecular motors through adaptor proteins. Different examples show that the same set of proteins, which mediate motor-cargo interactions of neurotransmitter receptors, have been previously described as components of postsynaptic densities (PSDs), where they mediate scaffolding reactions, such as for instance the clustering of receptors at axo-dendritic contacts. As discussed above, the proteins of the mLIN-family that connect the NMDA-type glutamate receptor subunit NR2B with KIF1712 as well as

GRIP1, which couples the AMPA-type glutamate receptor subunit GluR2 to KIF5 A, B and C6, locate and concentrate at postsynaptic sites and mediate specific protein-protein interactions at this membrane compartment. It has also been pointed out that for instance mLIN2, mLIN7, and mLIN10 harbor PDZ domains, known to interact with a variety of proteins, which enables such candidates to participate in the formation of larger protein complexes. Furthermore, the postsynaptic proteins gephyrin and PSD-95, known to participate in both transport and postsynaptic reactions, multimerize by self-interaction domains and thereby form protein scaffolds with multiple sites for protein-protein interactions32. In general, the use of a common set of proteins as both intracellular transport adaptors and synaptic scaffold proteins might contribute to the transport specificity and postsynaptic integration of receptors that underlie synapse formation and plasticity (Figure 13.3).

Figure 13.3. Proteins with Dual Functions in Motor-Protein-Dependent Transport and at Postsynaptic Membrane Specializations Contribute to the Post-Golgi Surface Membrane Delivery of Neurotransmitter Receptors. Two possibilities for the delivery of receptors are shown in (A) and (B). The currently available data favor the mechanism shown in (A).

Figure 13.3. Proteins with Dual Functions in Motor-Protein-Dependent Transport and at Postsynaptic Membrane Specializations Contribute to the Post-Golgi Surface Membrane Delivery of Neurotransmitter Receptors. Two possibilities for the delivery of receptors are shown in (A) and (B). The currently available data favor the mechanism shown in (A).

Neurotransmitter receptors could be directly or indirectly delivered to the synapse; however the maintenance of the receptor-adaptor interaction after transport could have several advantages for the entry and stabilization of receptors at existing postsynaptic specializations. With respect to the formation of presynaptic active zones, preformed large protein complexes have been described that are subject of common transport of precursor membrane specializations33. Similarly, a set of proteins, which functions in both transport and membrane scaffold formation, would be suitable to mediate directed delivery of preformed protein complexes that need to be maintained. It is currently unknown whether the choice of adaptor/scaffolding proteins mediates a certain contribution of transport specificity in individual transport complexes and whether this represents a specific phenomenon or rather a general principle.

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