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 α, β and γ isoforms share an epidermis-restricted expression pattern, whereas the δ isoform is intracellular and ubiquitous. To get an insight into Dmknδ function, we performed yeast two-hybrid screening and identified the small GTPases Rab5 as partners for Dmknδ. 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 Dmknδ. Transient expression of Dmknδ in HeLa cells led to the formation of punctate structures colocalized with endogenous Rab5 and clathrin, indicating Dmknδ involvement in the early steps of endocytosis. Dmknδ indeed colocalized with transferrin at early stages of endocytosis, but did not modulate its endocytosis or recycling kinetics. We also showed that Dmknδ 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, Dmknδ expression rescued the Rab5S34N-mediated inhibition of endosome fusion. Moreover, Dmknδ caused the enlargement of vesicles positive for Rab5 by promoting GTP loading onto the small GTPase. Together our data reveal that Dmknδ activates Rab5 function and thus is involved in the early endosomal trafficking.