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* Department of Biological Chemistry, and While the localization of chemoattractant receptors on randomly oriented cells has been previously
studied by immunohistochemistry, the instantaneous
distribution of receptors on living cells undergoing directed migration has not been determined. To do this,
we replaced cAR1, the primary cAMP receptor of Dictyostelium, with a cAR1-green fluorescence protein fusion construct. We found that this chimeric protein is
functionally indistinguishable from wild-type cAR1. By
time-lapse imaging of single cells, we observed that the
receptors remained evenly distributed on the cell surface and all of its projections during chemotaxis involving turns and reversals of polarity directed by repositioning of a chemoattractant-filled micropipet. Thus,
cell polarization cannot result from a gradient-induced asymmetric distribution of chemoattractant receptors.
Some newly extended pseudopods at migration fronts
showed a transient drop in fluorescence signals, suggesting that the flow of receptors into these zones may
slightly lag behind the protrusion process. Challenge
with a uniform increase in chemoattractant, sufficient to cause a dramatic decrease in the affinity of surface
binding sites and cell desensitization, also did not significantly alter the distribution profile. Hence, the induced
reduction in binding activity and cellular sensitivity
cannot be due to receptor relocalization. The chimeric
receptors were able to "cap" rapidly during treatment with Con A, suggesting that they are mobile in the
plane of the cell membrane. This capping was not influenced by pretreatment with chemoattractant.
Department of Cell Biology and Anatomy, School of Medicine, Johns Hopkins
University, Baltimore, Maryland 21205
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