Phospholamban (PLB) is a 52-amino acid inhibitor of the Ca2+-ATPase in cardiac sarcoplasmic reticulum (SERCA2a), which acts by decreasing the apparent affinity of the enzyme for Ca2+. To localize binding sites of SERCA2a for PLB, we performed Cys-scanning mutagenesis of PLB, co-expressed the PLB mutants with SERCA2a in insect cell microsomes, and tested for cross-linking of the mutated PLB molecules to SERCA2a using 1,6-bismaleimidohexane, a 10-Å-long, homobifunctional thiol cross-linking agent. Of several mutants tested, only PLB with a Cys replacement at position 30 (N30CoPLB) cross-linked to SERCA2a. Cross-linking occurred specifically and with high efficiency. The process was abolished by micromolar Ca2+ or by an anti-PLB monoclonal antibody and was inhibited 50% by phosphorylation of PLB by cAMP-dependent protein kinase. The SERCA2a inhibitors thapsigargin and cyclopiazonic acid also completely prevented cross-linking. The two essential requirements for cross-linking of N30C-PLB to SERCA2a were a Ca2+-free enzyme and, unexpectedly, a micromolar concentration of ATP or ADP, demonstrating that N30C-PLB cross-links preferentially to the nucleotide-bound, E2 state of SERCA2a. Sequencing of a purified proteolytic fragment in combination with SERCA2a mutagenesis identified Cys318 of SERCA2a as the sole amino acid cross-linked to N30C-PLB. The proximity of residue 30 of PLB to Cys318 of SERCA2a suggests that PLB may interfere with Ca2+ activation of SERCA2a by a protein interaction occurring near transmembrane helix M4.