A robust cross-link between Gln(23) in phospholamban (PLN) and Lys(328) in the sarco(endo)plasmic reticulum Ca2+ ATPase (SERCA1a) is formed in the presence or absence of oxidant and is susceptible to both PLN phosphorylation and SERCA1a Ca2+ binding. This cross-link provides precisely the evidence needed to support our earlier proposal that collision of the PLN transmembrane helix at Asn(27) with the cytosolic extension of M4 at Leu(321) leads to unwinding of the helix. In a study of site-specific interactions among PLN, sarcolipin (SLN), and SERCA1a, we determined that mutations of some specific amino acids in PLN or SLN diminish either the super-inhibition imposed on SERCA1a function by the PLN-SLN binary complex or the physical interactions between PLN and SLN or both. These results have led to a revision of our earlier model for the PLN-SLN-SERCA1a complex. (c) 2007 Elsevier Inc. All rights reserved.