FIGURE 6.

Ca²⁺ sensitivity of MPEG-5 reactivity in selected IP₃R mutants. A, MPEG-5 reaction of the endogenous cysteine in the 95-kDa C-terminal trypsin fragment of microsomes prepared from COS cells transfected with the indicated mutants was measured at different buffered free [Ca²⁺] by immunoblotting with the intraluminal IL-3 Ab. The position of the 95-kDa unshifted and shifted bands are shown by the open and closed arrows, respectively. The asterisk indicates the presence of a variable intensity, nonspecific band detected by the Ab. B, the magnitude of the shifted band was estimated densitometrically using low exposure blots and is expressed as a percentage of the maximum shift. The data are the mean of two experiments on independent microsome preparations. WT, wild-type.