Dynamics of KRas on endosomes: involvement of acidic phospholipids in its association

The endocytic compartment is emerging as a functional platform for controlling important cellular processes. We have found that similar to 10 to 15% of total KRas, a protein that is frequently mutated in cancer, is present on endosomes, independent of its activation state. The dynamics of GFP-KRas wild-type (WT) and constitutively active or inactive mutants on endosomes were analyzed by fluorescence recovery after photobleaching (FRAP) microscopy. The measurements revealed an extraordinarily fast recovery of KRas WT [half-time (HT), similar to 1.3 s] compared to HRas, Rab5, and EGFR, with the active KRasG12V mutant being significantly faster and more mobile (HT, similar to 1 s, and similar to 82% of exchangeable fraction) than the inactive KRasS17N (HT, similar to 1.6 s, and similar to 60% of exchangeable fraction). KRas rapidly switches from the cytoplasm to the endosomal membranes by an electrostatic interaction between its polybasic region and the endosomal acidic phospholipids, mainly phosphatidylserine.