The distribution of chemotactic peptide receptors on polymorphonuclear leukocytes (PMNs) was visualized using tritiated chemotactic peptide, N-formylmethionyl-leucylphenylalanine, coupled to hemocyanin (HY-FMLP). This probe was biologically active and the number of HY-FMLP molecules bound to the cell in a saturable manner corresponded closely to the number of peptide receptors characterized for rabbit peritoneal polymorphonuclear leukocytes (Sullivan, S. J., and S. H. Zigmond, 1980, J. Cell Biol., 85:703-711). Cells exhibiting locomotion have a polar morphology easily recognized in the scanning electron microscope. HY-FMLP bound to these cells was asymmetrically distributed with the highest density of HY-FMLP bound to the midregion of the cell. There were very few particles bound to the tail regions. The binding to the leading ruffles was variable but usually less than to the midregion. Addition of high concentrations of uncoupled FMLP eliminated HY-FMLP binding, confirming that the hemocyanin observed was a marker for the saturable chemotactic peptide receptor. The asymmetry in receptor distribution was seen on cells that had been stimulated by low concentrations of either FMLP or another chemotactic factor, leukotriene B4. Thus, peptide binding to the receptor was not required for the development of the asymmetric distribution. The low density of receptors in the tail region of the cell was consistent with the decreased responsiveness of the tail to chemotactic stimulation (Zigmond, S. H., H. I. Levitsky, and B. J. Kreel, 1981, J. Cell Biol., 89:585-592). The receptor asymmetry may contribute to the polar behavior exhibited by polymorphonuclear leukocytes and would be expected to quantitatively modify the directional information available to a cell in a gradient of chemotactic peptide.

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