Calcium Ion Regulation Antibody Sampler Kit

Background

Sarcoplasmic and endoplasmic reticulum Ca²⁺ ATPases (SERCA) are members of a highly conserved family of Ca²⁺ pumps (1). ATP2A1 (SERCA1) is a fast-twitch, skeletal muscle sarcoplasmic reticulum (SR) Ca²⁺ ATPase (2). Multiple ATP2A2 (SERCA2) isoforms have been isolated, with ATP2A2a (SERCA2a) found predominantly in the SR of muscle cells and ATP2A2b (SERCA2b) more ubiquitously expressed in the ER of most cell types (3). Post-translational modification of ATP2A2, including phosphorylation and tyrosine nitration, modify Ca²⁺ -ATPase activity and calcium transport (4,5).

Phospholamban (PLN) was identified as a major phosphoprotein component of the SR (6). Despite very high expression in cardiac tissue, phospholamban is also expressed in skeletal and smooth muscle (7). Localization of PLN is limited to the SR, where it serves as a regulator of the sarco-endoplasmic reticulum calcium ATPase, SERCA (8). PLN binds directly to SERCA and effectively lowers its affinity for calcium, thus reducing calcium transport into the SR. Phosphorylation of PLN at Ser16 by PKA or myotonic dystrophy protein kinase and/or phosphorylation at Thr17 by Ca²⁺/calmodulin-dependent protein kinase results in release of PLN from SERCA, relief of this inhibition, and increased calcium uptake by SR (reviewed in 9,10). It has long been held that phosphorylation at Ser16 and Thr17 occurs sequentially, but increasing evidence suggests that phosphorylation, especially at Thr17, may be differentially regulated (reviewed in 11,12).

The second messenger cyclic AMP (cAMP) activates cAMP-dependent protein kinase (PKA or cAPK) in mammalian cells and controls many cellular mechanisms such as gene transcription, ion transport, and protein phosphorylation (13). Inactive PKA is a heterotetramer composed of a regulatory subunit (R) dimer and a catalytic subunit (C) dimer. In this inactive state, the pseudosubstrate sequences on the R subunits block the active sites on the C subunits. Three C subunit isoforms (C-α, C-β, and C-γ) and two families of the regulatory subunits (RI and RII) with distinct cAMP binding properties have been identified. Upon binding of cAMP to the R subunits, the auto-inhibitory contact is eased and active monomeric C subunits are released. PKA shares substrate specificity with Akt (PKB) and PKC, which are characterized by an arginine at position -3 relative to the phosphorylated serine or threonine residue (14). PKA phosphorylation is involved in the regulation of Ca²⁺ channels, including Ca_v1.1 in skeletal muscle and Ca_v1.2 in the heart (reviewed in 15).