Small calcium binding protein that plays important roles in several biological processes such as Ca(2+) homeostasis, chondrocyte biology and cardiomyocyte regulation (PubMed:12804600). In response to an increase in intracellular Ca(2+) levels, binds calcium which triggers conformational changes (PubMed:23351007). These changes allow interactions with specific target proteins and modulate their activity (PubMed:22399290). Regulates a network in cardiomyocytes controlling sarcoplasmic reticulum Ca(2+) cycling and mitochondrial function through interaction with the ryanodine receptors RYR1 and RYR2, sarcoplasmic reticulum Ca(2+)-ATPase/ATP2A2 and mitochondrial F1-ATPase (PubMed:12804600). Facilitates diastolic Ca(2+) dissociation and myofilament mechanics in order to improve relaxation during diastole (PubMed:11717446). Dimer of either two alpha chains, or two beta chains, or one alpha and one beta chain (PubMed:21296671). Also forms heterodimers with S100P (PubMed:15171681). Interacts with AGER (By similarity). Interacts with CAPZA1 (By similarity). Interacts with FKBP4 (PubMed:20188096). Interacts with RYR1 and RYR2 (PubMed:18650434). Interacts with CACYBP in a calcium-dependent manner (PubMed:12042313). Interacts with PPP5C (via TPR repeats); the interaction is calcium-dependent and modulates PPP5C activity (PubMed:22399290). Interacts with ATP2A2 and PLN in a Ca(2+)-dependent manner (PubMed:12804600). Interacts with mitochondrial F1-ATPase subunits ATP5F1A and ATP5F1B; these interactions increase F1-ATPase activity (By similarity). Highly prevalent in heart (PubMed:12804600, PubMed:1384693). Also found in lesser quantities in skeletal muscle and brain (PubMed:1384693). Glutathionylated; glutathionylation increases affinity to calcium about 10-fold. Able to bind zinc in vitro; the binding sites are different from the calcium binding sites. The physiological relevance of zinc binding is unclear. Physiological concentrations of potassium antagonize the binding of both divalent cations, especially affecting the high-affinity calcium-binding sites. Belongs to the S-100 family.