Ankyrin-B is required for intracellular sorting of structurally diverse Ca2+ homeostasis proteins

This report describes a congenital myopathy and major loss of thymic lympho cytes in ankyrin-B (-/-) mice as well as dramatic alterations in intracellu lar localization of key components of the Ca2+ homeostasis machinery in ank yrin-B (-/-) striated muscle and thymus. The sacoplasmic reticulum (SR) and SR/T-tubule junctions are apparently preserved in a normal distribution in ankyrin-B (-/-) skeletal muscle based on electron microscopy and the prese nce of a normal pattern of triadin and dihydropyridine receptor, Therefore, the abnormal localization of SR/ER Ca ATPase (SERCA) and ryanodine recepto rs represents a defect in intracellular sorting of these proteins in skelet al muscle, Extrapolation of these observations suggests defective targeting as the basis for abnormal localization of ryanodine receptors, IP3 recepto rs and SERCA in heart, and of IP3 receptors in the thymus of ankyrin-B (-/- ) mice. Mis-sorting of SERCA 2 and ryanodine receptor 2 in ankyrin-B (-/-) cardiomyocytes is rescued by expression of 220-kD ankyrin-B, demonstrating that lack of the 220-kD ankyrin-B polypeptide is the primary defect in thes e cells. Ankyrin-B is associated with intracellular vesicles, but is not co localized with the bulk of SERCA I or ryanodine receptor type 1 in skeletal muscle. These data provide the first evidence of a physiological requireme nt for ankyrin-B in intracellular targeting of the calcium homeostasis mach inery of striated muscle and immune system, and moreover, support a catalyt ic role that does not involve permanent stoichiometric complexes between an kyrin-B and targeted proteins. Ankyrin-B is a member of a family of adapter proteins implicated in restriction of diverse proteins to specialized plas ma membrane domains. Similar mechanisms involving ankyrins may be essential for segregation of functionally defined proteins within specialized region s of the plasma membrane and within the Ca2+ homeostasis compartment of the ER.