“…Techniques assessing protein-protein interactions have included co-immunoprecipitation (11), chemical crosslinking (12,13,17), FRET (14,16,18), and two-dimensional * This work was supported, in whole or in part, by National Institutes of Health Grant HL49428. 1 To whom correspondence should be addressed: Krannert Institute of Cardiology, 1800 N. Capitol Ave., Indianapolis, IN 46202. co-crystallization (2,19), which have frequently given conflicting results. For example, some studies (including our own) suggest that the Ca 2ϩ pump with bound PLB is catalytically inactive, requiring PLB dissociation prior to enzyme activation (11,12,13,15,17), whereas others maintain that PLB is a subunit of the Ca 2ϩ pump that remains attached throughout the catalytic cycle (14,16,18).…”