Calcium-activated ATPase of the chick embryonic chorioallantoic membrane. Identification, developmental expression, and topographic relationship with calcium-binding protein.

Abstract

A Ca2+-activated ATPase activity is present in the chick embryonic chorioallantoic membrane (CAM), the placenta-like tissue which translocates eggshell calcium into the embryonic circulation. The enzyme is membrane-bound, ATP-specific, Mg2+-dependent, exhibits dual Km values of 30 microM and 0.3 mM Ca2+, and has a Mr of 170,000. Throughout embryonic development, a single electrophoretic form of the Ca2+-ATPase is found and, furthermore, its specific activity as a function of age follows a bimodal pattern. In particular, from incubation days 14-15 to the end of gestation, a period representing rapid embryonic calcium accumulation, Ca2+-ATPase specific activity increases 6-fold. Cytohistochemistry localized the Ca2+-ATPase exclusively within the CAM ectoderm which lies adjacent to the calcium-rich shell membrane/eggshell. In a parallel study, cleavable bifunctional cross-linking agents were used to characterize the in situ protein topography of the CAM ectodermal surface adjacent to the calcium-binding protein (CaBP), a CAM cell-surface protein associated with calcium transport. We found that the immediate near neighbor of the CaBP is a 170,000 Mr, membrane-bound protein. The 170,000 protein was co-isolated with the CaBP after cross-linkage in situ and subsequent immunoprecipitation with anti-CaBP antibodies. Reductive cleavage of the immune complex released detectable Ca2+-ATPase activity, suggesting that the 170,000 protein is the Ca2+-ATPase of the CAM.