Protein found in or associated with endosomes. Endosomes are highly dynamic membrane systems involved in transport within the cell, they receive endocytosed cell membrane molecules and sort them for either degradation or recycling back to the cell surface. They also receive newly synthesised proteins destined for vacuolar/lysosomal compartments. In certain cell types, endosomal multivesicular bodies may fuse with the cell surface in an exocytic manner. These released vesicles are called exosomes.