Membrane topology as revealed by the binding of macromolecules

Binding sites for specific molecules at the cell surface can be localized with the electron microscope in thin-section, freeze-etch and shadow-casting preparations. The receptors tested were binding sites to concanavalin A (Con A) labelled with haemocyanin and binding sites to insulin labelled with ferritin. Con A-binding sites were localized in endocrine cells of the pancreas and insulin-binding sites in isolated liver plasma membranes. The relevance of the topographical distribution of the binding sites to cell membrane organization is discussed.