Densin-180 forms a ternary complex with the α-subunit of Ca2+/calmodulin-dependent protein kinase II and α-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 alpha -subunit of Ca2+/calmodulin-dependent protein kinase II (CaMKII) and alpha -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 alpha -subunit of CaMKII (CaMKII alpha), and alpha -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 CaMKII alpha and does not bind with high affinity to holoenzymes of CaMKII that contain beta -subunit. The PDZ (PSD-95, DIg, Z0-1) domain of densin contributes to its binding to alpha -actinin. A distinct domain of alpha -actinin interacts with the kinase domains of both alpha and beta -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 densin-180 by CaMKII at serine-1397 only slightly decreases its affinity for CaMKII. The specific interaction of densin-180 with holoenzymes of CaMKII containing only alpha -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.