Different Crosslinking Agents Identify Distinctly Different Putative Escherichia Coli Heat-Stable Enterotoxin Rat Intestinal Cell Receptor Proteins

Abstract

The receptor for heat-stable enterotoxins (ST) produced by Escherichia coli and related organisms is located in the brush border region of intestinal villus cells. Heterobifunctional and homobifunctional crosslinkers were used to covalently couple ¹²⁵I-ST to rat intestinal cell brush border membrane proteins. Experimental conditions during ligand binding and subsequent crosslinking significantly influence the efficiency of crosslinking, and the number of peptides specifically crosslinked to the ¹²⁵I-ST. Multiple proteins efficiently coupled to ¹²⁵I-ST with agents that can couple through the ST amino terminus. The crosslinker 1-ethyl-3-(dimethylaminopropyl)carbodiimide (EDC), which can react with the carboxy terminus of the ST, covalently crosslinked ¹²I-ST to a single protein with an apparent Mr of 125,000–130,000, larger than the proteins identified using longer crosslinkers. Each of the proteins identified by crosslinking migrate with the same retention time on gel filtration after solubilization, with an approximate molecular size of 150,000–200,000.