Receptor-mediated endocytosis of a ricin-colloidal gold conjugate in vero cells. Intracellular routing to vacuolar and tubulo-vesicular portions of the endosomal system.

We have prepared a conjugate (Ri-Au) of the toxic plant protein ricin and colloidal gold (particle size 5 nm) and used it for internalization studies in monolayer cultures of Vero cells. The Ri-Au conjugate was very stable, with only little release of ricin ([125I]Ri) from the gold particles within a pH range of 4.5-8.0. Within 2 h at 37 degrees C, only very little intracellular degradation of the ricin preparation ([125I]Ri-Au) occurred. The cells bound the same proportion of native ricin ([125I]Ri) and Ri-Au from the medium, and the kinetics of toxicity (decrease in cellular incorporation of [3H]leucine) of [125I]Ri and [125I]Ri-Au were also comparable. At 4 degrees C, the cell-surface binding of Ri-Au was continuous and distinct, as revealed by electron microscopy. This binding was specific, since almost no Ri-Au surface binding occurred at 4 degrees C in the presence of 0.1 M lactose or 1 mg/ml native (unlabelled) ricin. Within the first 30 min of warming prelabelled cells to 37 degrees C, the amount of surface-associated Ri-Au decreased considerably (from 150 to 60 gold particles per micron cell surface in 40 nm sections). Coated pits and vesicles were involved in the internalization of Ri-Au, and within 5-30 min at 37 degrees C Ri-Au had been delivered to vacuolar and tubulo-vesicular portions of the endosomal system, and later also to lysosomes. Analysis of very thin (ca 20 nm) serial sections revealed that most of the tubulo-vesicular elements were separate structures not connected to the membrane of the vacuolar portion. Data here presented indicate that our ricin conjugate, like many "physiological' ligands and viruses, is internalized by receptor-mediated endocytosis via the coated pit-endosomal pathway.