Binding of mannosylated ferritin to chicken bone marrow macrophages.

Ferritin conjugated to diazo p-aminophenylmannoside and mannan was used for ultrastructural visualization of binding and endocytosis via the mannose receptor. Conjugates were bound by live macrophages but not by glutaraldehyde-fixed cells. Binding was inhibited by 0.1 mM mannan and 0.2 M alpha-methylmannoside, and strongly reduced, but not abolished, after trypsin degradation of surface receptors. Binding sites were rapidly assembled on coated pits, which often showed a polar distribution on the cell surface and entered the cell following the direction of microtubuli. Some coated vesicles occasionally kept in touch with the extracellular space. 10-min exposure to the conjugate resulted in changes in the surface morphology, such as a loss of infoldings and increased membrane tension. Ferritin accumulated in large smooth vesicles with coated membrane regions. It was mostly detached from the membrane and agglutinated into big clumps, together with fibrillar material and small vesicles which may derive from the vesicle membrane.