In this study we investigated the function of Sorting Nexin 3 (SNX3) in the formation of intralumenal vesicles (ILVs). SNX3 is a small protein containing a particular PX domain that recognizes PI3P, hence the protein is located on the early endosomes, where it regulates ILV formation and retrograde transport of some cargoes. We investigated the role of SNX3 homologue SNX12 in the endocytic pathway and found that these proteins share redundant functions on ILV formation. In collaboration with Michael Overduin and Marc Lenoir we elucidated the solution structure of full-length SNX3 and identified a serine residue (Ser72) at the rim of the PI3P recognition pocket, which prevents PI3P binding once it is phosphorylated. Conserved nature of this serine residue among the SNX Family members suggests phosphorylation as a regulatory mechanism for PI3P-binding PX domains. To identify the signaling pathways involved in regulation of SNX3, we developed a kinase inhibitory library screen using HeLa cells stably expressing GFP-SNX3. Our screen revealed several potential targets, which regulate amount of GFP-SNX3 on endosomes.