Rotational diffusion of epidermal growth factor complexed to cell surface receptors reflects rapid microaggregation and endocytosis of occupied receptors.

The rotational diffusion of epidermal growth factor (EGF)--receptor complexes on living human epidermoid carcinoma cells (A-431) has been measured by phosphorescence emission and anisotropy in the mu s time domain. A biologically active phosphorescent conjugate of EGF, erythrosin-EGF, was applied to living cells. The hormone--receptor complexes were mobile with rotational correlation times in the range 25--90 mu s when labeled and measured at 4 degrees C. Prolonged incubation and exposure to higher temperatures (23 and 37 degrees C) resulted in longer times up to 350 mu s, indicative of the progressive formation of microclusters, estimated to contain 10-50 receptors. Upon internalization of the hormone--receptor complexes, visible patches were observed by fluorescence microscopy, and the rotational correlation times were shorter, indicating a decrease in size of the dynamic unit. The sign of the rotational relaxation also varied with localization and processing of the hormones. The rate of lateral diffusion of EGF--receptor complexes, measured under similar conditions by fluorescence photobleaching recovery, increased with temperature in contrast to the rotational motion.