CTP:phosphocholine cytidylyltransferase (CCTa) is a proteolytically-sensitive enzyme essential for production of phosphatidylcholine, the major phospholipid of animal cell membranes. The molecular signals that govern CCTa protein stability are unknown. An NH₂-terminal PEST sequence within CCTa did not serve as a degradation signal for the proteinase, calpain. Calmodulin (CaM) stabilized CCTa from calpain proteolysis. Adenoviral gene transfer of CaM in cells protected CCTa whereas CaM siRNA accentuated CCTa degradation by calpains. CaM bound CCTa as revealed by FRET and 2-hybrid analysis. Mapping and site-directed mutagenesis of CCTa uncovered a motif (LQERVDKVK) harboring a vital recognition site, Q243, whereby CaM directly binds to the enzyme. Mutagenesis of CCTa Q243 not only resulted in loss of CaM binding, but also led to complete calpain resistance in vitro and in vivo. Thus, calpains and CaM both access CCTa using a structurally similar molecular signature that profoundly affects CCTa levels. These data suggest that CaM, by antagonizing calpain, serves as a novel binding partner for CCTa that stabilizes the enzyme under pro-inflammatory stress.