SERCA2a activity is regulated by phosphorylation of another SR protein: Phospholamban (PLN). Dephosphorylated PLN inhibits SERCA2a. Phosphorylation of PLN by either cAMP or cGMP-dependent protein kinase at Ser16 or the Ca2+-calmodulin-dependent protein kinase (CaMKII), at Thr17, relieves this inhibition, increasing SR Ca2+ uptake and SR Ca2+ load.