ENDOCYTOSIS AND LYSOSOMAL TARGETING OF EPIDERMAL GROWTH-FACTOR RECEPTORS ARE MEDIATED BY DISTINCT SEQUENCES INDEPENDENT OF THE TYROSINE KINASE DOMAIN

Ligand-induced internalization of the epidermal growth factor receptor (EGFR) leads to accelerated receptor degradation, Two models have bee n proposed to explain this. In the first model, induced internalizatio n expands the intracellular pool of receptors, leading to enhanced lys osomal targeting. The second model proposes that activation of intrins ic receptor kinase activity induces inward vesiculation of endosomes, thus interrupting receptor recycling. To test these models, we created EGFR mutants that lack the conserved tyrosine kinase domain, but reta in different parts of the distal carboxyl terminus regulatory region. Mutants lacking all distal regulatory sequences underwent slow interna lization (0.02 min(-1)) and turnover (t(1/2) similar to 24 h), similar to unoccupied, holo-EGFR. Mutant receptors that lacked the kinase dom ain, but retained the entire distal regulatory domain, were constituti vely internalized and targeted to lysosomes, even in the absence of EG F. The turnover of these receptors (t(1/2) similar to 11 h) was simila r to that of occupied, kinase-active holo-EGFR (t(1/2) similar to 9.5 h). These results show that receptor tyrosine kinase activity is not r equired for the targeting of EGFR to lysosomes, Receptor mutants which expressed previously identified endocytic sequences underwent rapid i nternalization. Unexpectedly, enhanced turnover of EGFR mutants requir ed additional sequences located between residues 945 and 991 in the ho lo-EGFR. Thus, internalization and lysosomal targeting of EGFR are sep arate processes mediated by distinct sequences. Our results indicate t hat induced internalization is necessary, but not sufficient, for enha nced EGFR degradation. Instead, down-regulation requires exposure of p reviously cryptic internalization and lysosomal targeting sequences. O ccupied EGFR thus appear to be handled by the endocytic machinery in t he same fashion as other constitutively internalized or lysosomally ta rgeted receptors.