ENDOCYTOSIS OF FOLATE-PROTEIN CONJUGATES - ULTRASTRUCTURAL-LOCALIZATION IN KB CELLS

It has been demonstrated that proteins covalently conjugated to folic acid may be taken up by cells via cytosis after binding to a folate bi nding protein (FBP) in the cell membrane. The proteins taken up in thi s manner remain catalytically active and they may modify physiological processes occurring in the cytosol. Confocal fluorescence microscopy of KB cells incubated with FITC-bovine serum albumin-folic acid conjug ates showed that after uptake, the conjugates resided in large vesicul ar structures. The purpose of the present study was to determine the s ubcellular localization of protein-folic acid conjugates in KB cells u sing folic acid-bovine serum albumin-colloidal gold (F-BSA-CG) as a tr acer. F-BSA-CG conjugates were taken up via uncoated pits or caveolae, and resided primarily in multivesicular bodies (MVBs) and other tubul ar endosomes at early time points (15-60 min). At later time points (6 hours), conjugates were still contained in MVBs but some were also fo und in secondary lysosomes or free in the cytoplasm. Coincubation of K B cells with transferrin-colloidal gold (TF-CG) and F-BSA-CG resulted in colocalization of TF-CG. and F-BSA-CG within endosomal elements at times later than 15 minutes, indicating that the caveolae-mediated F-B SA-CG endocytic pathway converged with a pathway utilized by clathrin- coated pits.