The Ubiquitous Dermokine Delta Activates Rab5 Function in the Early Endocytic Pathway

The expression of the recently identified dermokine (Dmkn) gene leads to four families of proteins with as yet unknown functions. The secreted alpha, beta and gamma isoforms share an epidermis-restricted expression pattern, whereas the delta isoform is intracellular and ubiquitous. To get an insight into Dmkn delta function, we performed yeast two-hybrid screening and identified the small GTPases Rab5 as partners for Dmkn delta. The Rab5 proteins are known to regulate membrane docking and fusion in the early endocytic pathway. GST pull-down assays confirmed the direct interaction between Rab5 and Dmkn delta. Transient expression of Dmkn delta in HeLa cells led to the formation of punctate structures colocalized with endogenous Rab5 and clathrin, indicating Dmkn delta involvement in the early steps of endocytosis. Dmkn delta indeed colocalized with transferrin at early stages of endocytosis, but did not modulate its endocytosis or recycling kinetics. We also showed that Dmkn delta was able to bind both inactive (GDP-bound) and active (GTP-bound) forms of Rab5 in vitro but preferentially targeted GDP-bound form in HeLa cells. Interestingly, Dmkn delta expression rescued the Rab5S34N-mediated inhibition of endosome fusion. Moreover, Dmkn delta caused the enlargement of vesicles positive for Rab5 by promoting GTP loading onto the small GTPase. Together our data reveal that Dmkn delta activates Rab5 function and thus is involved in the early endosomal trafficking.