Protein-gold transport in the endocytic complex of trophotaenial absorptive cells in the embryos of a goodeid teleost.

This paper reports adsorptive endocytosis of exogenous proteins by the trophotaenial absorptive cells (TACs) in the viviparous goodeid teleost, Ameca splendens. In vitro incubations were performed with gold conjugated to bovine serum albumin (Au-BSA), human transferrin (Au-HTf), fetuin (Au-Fet), and asialofetuin (Au-ASFet). Localization of gold label on the TAC surface was nearly exclusive to patches of an amorphous coat associated with part of the intermicrovillous plasma membrane. On addition of excess BSA, HTf, Fet, or ASFet to incubation media containing, respectively, Au-BSA, Au-HTf, Au-Fet, or Au-ASFet, the density of gold particles adsorbed on the TAC surface decreased drastically. Moreover, attachment of the four protein-gold complexes to the same plasma membrane sites was suggested by reciprocal inhibitory effects. Further proteins such as hemoglobin, myoglobin, and cytochrome c were as well potent inhibitors of Au-BSA and Au-HTf binding and uptake. Binding of TACs of native BSA or HTf was visualized by immunogold labeling. The interactions between proteins and binding sites required both the presence of Ca2+ and appropriate pH greater than 6.6. Analyses of the concentration-dependent BSA and HTf binding curves, plotted from morphometric data, resulted in apparent dissociation constants, Kds, of approximately 5 x 10(-7) M and 4 x 10(-7) M, respectively. Following binding at the TAC surface and internalization via clathrin-coated pits and vesicles the several ligands were routed along the lysosomal pathway with transit through the endosomal compartment. Prolonged incubation periods led to massive intracellular accumulation of tracer proteins. The effects of NH4Cl (10 mM) treatment on TACs included enormous cytoplasmic vacuolation, a reversible loss of protein binding sites on the plasma membrane, and a block in the transport of protein-gold complexes to lysosomes.