Calcium/calmodulin-dependent protein kinase that functions autonomously after Ca(2+)/calmodulin-binding and autophosphorylation, and is involved in dendritic spine and synapse formation, neuronal plasticity and regulation of sarcoplasmic reticulum Ca(2+) transport in skeletal muscle. In neurons, plays an essential structural role in the reorganization of the actin cytoskeleton during plasticity by binding and bundling actin filaments in a kinase-independent manner. This structural function is required for correct targeting of CaMK2A, which acts downstream of NMDAR to promote dendritic spine and synapse formation and maintain synaptic plasticity which enables long-term potentiation (LTP) and hippocampus-dependent learning. In developing hippocampal neurons, promotes arborization of the dendritic tree and in mature neurons, promotes dendritic remodeling. Also regulates the migration of developing neurons. Participates in the modulation of skeletal muscle function in response to exercise. In slow-twitch muscles, is involved in regulation of sarcoplasmic reticulum (SR) Ca(2+) transport and in fast-twitch muscle participates in the control of Ca(2+) release from the SR through phosphorylation of triadin, a ryanodine receptor-coupling factor, and phospholamban (PLN/PLB), an endogenous inhibitor of SERCA2A/ATP2A2. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Activated by Ca(2+)/calmodulin. Binding of calmodulin results in conformational change that relieves intrasteric autoinhibition and allows autophosphorylation of Thr-287 which turns the kinase in a constitutively active form and confers to the kinase a Ca(2+)-independent activity. CAMK2 is composed of 4 different chains: alpha (CAMK2A), beta (CAMK2B), gamma (CAMK2G), and delta (CAMK2D). The different isoforms assemble into homo- or heteromultimeric holoenzymes composed of 12 subunits with two hexameric rings stacked one on top of the other (By similarity). Interacts with SYNGAP1, CAMK2N2 and MPDZ (PubMed:9724800, PubMed:15312654). Interacts with FOXO3 (By similarity). Interacts (when in a kinase inactive state not associated with calmodulin) with ARC; leading to target ARC to inactive synapses (PubMed:22579289). Interacts with CAMK2N1; this interaction requires CAMK2B activation by Ca(2+) (PubMed:11182241). Colocalizes with the cortical actin cytoskeleton by binding to F-actin in hippocampal cells. By cocaine in cardiomyocytes. The CAMK2 protein kinases contain a unique C-terminal subunit association domain responsible for oligomerization. Autophosphorylation of Thr-287 following activation by Ca(2+)/calmodulin. Phosphorylation of Thr-287 locks the kinase into an activated state. Belongs to the protein kinase superfamily. CAMK Ser/Thr protein kinase family. CaMK subfamily.