Densin-180 Forms a Ternary Complex with the a-Subunit of Ca/Calmodulin-Dependent Protein Kinase II and a-Actinin

Densin-180 is a transmembrane protein that is tightly associated with the postsynaptic density in CNS neurons and is postulated to function as a synaptic adhesion molecule. Here we report the identification of the a-subunit of Ca/ calmodulin-dependent protein kinase II (CaMKII) and a-actinin-4 as potential binding partners for the densin-180 intracellular segment. We demonstrate by yeast two-hybrid and biochemical assays that the intracellular portion of densin-180, the a-subunit of CaMKII (CaMKIIa), and a-actinin interact with each other at distinct binding sites and can form a ternary complex stabilized by multiple interactions. Densin-180 binds specifically to the association domain of CaMKIIa and does not bind with high affinity to holoenzymes of CaMKII that contain b-subunit. The PDZ (PSD-95, DIg, Z0-1) domain of densin contributes to its binding to a-actinin. A distinct domain of a-actinin interacts with the kinase domains of both aand b-subunits of CaMKII. Autophosphorylation of CaMKII increases its affinity for densin-180 from an EC50 of .1 mm to an EC50 of ,75–150 nM. In contrast, phosphorylation of densin180 by CaMKII at serine-1397 only slightly decreases its affinity for CaMKII. The specific interaction of densin-180 with holoenzymes of CaMKII containing only a-subunit and the increased affinity of CaMKII for densin-180 after autophosphorylation suggest that densin-180 may be involved in localization of activated CaMKII synthesized in dendrites.