Abstract
In the goodeid placental analogue, trophotaeniae provide extraembryonic gut‐derived exchange surfaces. Ameca splendens embryos possess endocytosing trophotaeniae that are capable of absorbing a dazzling array of proteinaceous substances. The iron core protein, native ferritin (NF), and several radioiodinated proteinaceous substances were used to study ligand and binding site pathways in the trophotaenial absorptive cells (TACs). Time sequence analysis of NF trafficking indicated an exclusively lysosomal pathway. Binding to TACs of NF was completely inhibitable by proteins containing multiple lysine residues such as apoferritin, bovine serum albumin (BSA), human transferrin (HTf), fetuin, hemoglobin, myoglobin, cytochrome c, ubiquitin, parvalbumin as well as the random copolymers, poly(Glu,Lys,Tyr)6:3:1 and poly(D‐Glu,D‐Lys)6:4. Peptide hormones and pepsin that contains only one lysine residue did not produce inhibitory effects. Radiolabels such as ^125^I‐BSA, ^125^I‐HTf and ^125^I‐poly(Glu,Lys,Tyr) bound to trophotaeniae in a specific saturable manner. Any two proteins were shown to hinder one another in getting hold of a binding site. Concentration‐dependent ^125^I‐BSA binding and Scatchard analysis of the data revealed both low‐ and medium‐affinity binding with apparent dissociation constants, K~d~s, of 3.4×10^−5^ M and 2×10^−7^ M, respectively. Binding of NF and radioiodinated proteins was inhibited in the presence of a large excess of L‐Lys, D‐Lys, and several dipeptides containing Lys. Both Ca^2+^‐depletion and low pH dramatically reduced the TACs' capacity to bind proteins. The effects of acidotropic agents included a reversible loss of surface protein binding sites, tremendous vacuolation, and the arrest of lysosomal degradation. Collectively, present results demonstrate that TACs bind and absorb multiple proteinaceous substances through a mechanism satisfying the criteria of receptor‐mediated endocytosis. It is concluded that scavenger protein binding sites are used to ingest proteins for lysosomal degradation, helping to meet the embryos' amino acid requirement. J. Exp. Zool. 299A:197–212, 2003. © 2003 Wiley‐Liss, Inc.