Receptor-mediated Internalization of Tuftsin“ zyx ANDREW A. AMOSCATO,bC PETER J. A. DAVIES,’.d GEORGE F. BABCOCKb,‘ AND KENJl NISHIOKAb.‘ bDepartment of General SurgerylSurgical Research Laboratory and Department zyxw of Biochemistry The University of Texas System Cancer Center M. D. Anderson Hospital and Tumor lnstitute at Houston Houston, Texas 77030 ‘The University of Texas Health Science Center at Houston Graduate School of Biomedical Sciences and dDepartment of Pharmacology The University of Texas Medical School Houston, Texas 77030 zyxw INTRODUCTION One of the major problems in cell biology is the understanding of the mechanism by which extracellular ligands interact with their specific cell surface receptors to regulate intracellular metabolic events. It is now known that many ligands that bind to their specific receptors are rapidly internalized. This process, termed receptor- mediated endocytosis, has become recognized as the general mechanism by which cells acquire the necessary nutritional and regulatory proteins from their extracellular environment.’” Some biologically important molecules known to be taken up by this mechanism include transport proteins such as low-density lip~protein,~ and transcobal- amin zyxwvut 115 as well as effector molecules such as insulin,”’ epidermal growth factory and the N-formyl chemotactic peptide.” In many cases, rapid internalization of receptor-bound ligands is achieved through the clustering of receptors from an initial diffuse distribution. The total process may be accomplished within minutes at physiological temperatures, as is the case with the N-formyl chemotactic peptide bound to human granulocytes.” In addition to having receptors for the N-formyl chemotactic peptide, human granulocytes are known to possess specific receptors for tuftsin, an immunopotentiat- ing tetrapeptide of the sequence L-Thr-L-Lys-L-Pro-L-Arg.” Since the mechanism of action of tuftsin following binding to its receptor and the fate of the tuftsin molecule itself were not known, it was in our interest to delineate the early stages of tuftsin action following receptor occupancy. To accomplish this, a biologically active fluores- cent analogue of tuftsin was prepared so we could visualize its interaction on human granulocytes and monocytes using video intensification microscopy (VIM). This study shows that tuftsin receptors are initially distributed diffusely and relatively homoge- neously on the cell surface, then rapidly redistribute to form clusters that are then internalized. ‘This work was supported in part by grants CA27330 awarded by NCI, DHS and ROI AM27078 awarded by NIH. zyxwvu 114