[Purification and characterization of the membrane associated calmodulin from the human placenta]

The protein with the most predominant Ca2+ binding activity in membrane fractions of the human placenta was purified to electrophoretical homogeneity. This protein was identified as the membrane bound form of calmodulin based on the immunological cross reactivity and a number of biochemical properties. The membrane fraction of the human placenta contained 8.3% of the total cellular calmodulin activity. Approximately 2/3 of the membrane associated calmodulin was extracted by incubation with a Ca2+ chelating agent, EGTA, suggesting that this form of calmodulin is bound reversibly to membrane proteins in a Ca2+ dependent manner. However, the rest of the membrane associated calmodulin could not be extracted by EGTA treatment. The EGTA nonextractable calmodulin appears to be embedded in the membrane matrix.