Newly synthesized transferrin receptors can be detected in the endosome before they appear on the cell surface.

It is well established that a proportion of newly synthesized lysosomal enzymes and class II major histocompatibility complex antigens are delivered directly to the endocytic pathway from the Golgi complex. Here we show that a significant proportion of newly synthesized transferrin receptors can be detected in endosomes before reaching the cell surface. These newly synthesized transferrin receptors are delivered to the endosome more efficiently than either constitutively secreted soluble proteins or glycophosphatidylinositol-anchored plasma membrane proteins suggesting that their transfer to the endosome is signal-dependent. Identification of a signal-dependent transfer step for proteins like the transferrin receptor operating on the exocytic pathway has important implications for membrane biogenesis, especially in the establishment of cell surface polarity.