Proteins

presence ofonly the 37,39-kD transducin a, ,B doublet on immunoblots. Quantification of transducin:rhodopsin ratios for purified rat ROS demonstrated that the concentration ofa and transducin subunits did not vary during the daily light:dark cycle (Fig. 1 and Table 1). In addition, no soluble pool of transducin was preferentially lost during preparation of darkor light-adapted ROS. In contrast, the major dark-soluble ROS protein, 48K (11), was reduced in concentration in ROS membranes prepared from dark-adapted retinas (Fig. 1, b through d). Thus, massive depletion of transducin polypeptides from ROS did not occur at any time during the daily light-dark cycle. Explanations other than subunit translocation must account for the twoto fourfold differ-