Background information: ARAP1 is an Arf (ADP-ribosylation factor)-directed GAP (GTPase-activating protein) that inhibits the trafficking of EGFR (epidermal growth factor receptor) to the early endosome. To further understand the function of ARAP1, we sought to identify proteins that interact with ARAP1.
Results: Here we report that ARAP1 associates with the CIN85 (Cbl-interacting protein of 85 kDa). Arg86 and Arg90 of ARAP1 and the SH3 (Src homology 3) domains of CIN85 are necessary for the interaction. We found that a mutant of ARAP1 with reduced affinity for CIN85 does not efficiently rescue the effect of reduced ARAP1 expression on EGFR trafficking to the early endosome. Reduced expression of CIN85 has a similar effect as reduced expression of ARAP1 on traffic of the EGFR. Cbl proteins regulate the endocytic trafficking of the EGFR by mediating ubiquitination of the EGFR. Overexpression of ARAP1 reduced ubiquitination of the EGFR by Cbl and slowed Cbl-dependent degradation of the EGFR. Reduced expression of ARAP1 accelerated degradation of EGFR but did not affect the level of ubiquitination of the receptor that was detected.
Conclusion: ARAP1 interaction with CIN85 regulates endocytic trafficking of the EGFR and affects ubiquitination of EGFR. We propose a model in which the ARAP1-CIN85 complex drives exit of EGF-EGFR-Cbl complex from a pre-early endosome into a pathway distinct from the early endosome/lysosome pathway.