Interactions of Tom1L1 with the multivesicular body sorting machinery

Tom1L1 ( Tom1-like1) and related proteins Tom1 ((T) under bar arget (o) under barf (M) under bar yb1) and Tom1L2 ( Tom1- like2) constitute a new protein family characterized by the presence of a VHS ((V) under bar ps27p/ (H) under bar rs/(S) under bar tam) domain in the N- terminal portion followed by a GAT ((G) under bar GA (a) under bar nd (T) under bar om) domain. Recently it was demonstrated that the GAT domain of both Tom1 and Tom1L1 binds ubiquitin, suggesting that these proteins might participate in the sorting of ubiquitinated proteins into multivesicular bodies ( MVBs). Here we report a novel interaction between Tom1L1 and members of the MVB sorting machinery. Specifically, we found that the VHS domain of Tom1L1 interacts with Hrs ( (H) under bar epatocyte growth factor-(r) under bar egulated tyrosine kinase (s) under bar ubstrate), whereas a PTAP motif, located between the VHS and GAT domain of Tom1L1, is responsible for binding to TSG101 ((t) under bar umor (s) under bar usceptibility gene 101). Myc epitopetagged Tom1L1 showed a cytosolic distribution but was recruited to endosomes following Hrs expression. In addition, Tom1L1 possesses several tyrosine motifs at the C-terminal region that mediate interactions with members of the Src family kinases and other signaling proteins such as Grb2 and p85. We showed that a fraction of Fyn kinase localizes at endosomes and that this distribution becomes more evident after epidermal growth factor internalization. Moreover, expression of a constitutive active form of Fyn also promoted the recruitment of Tom1L1 to enlarged endosomes. Taken together, we propose that Tom1L1 could act as an intermediary between signaling and degradative pathways.