Functional co-localization of monocytic aminopeptidase N/CD13 with the Fcγ receptors CD32 and CD64

Information about the function of aminopeptidase N/CD13 on monocytes is limited. In order to gain more insight into its interaction with other proteins, we have identified molecules that co-localize with the membrane ectoenzyme at the cell surface of monocytes. Using laser scanning and electron microscopy as well as fluorescence resonance energy transfer (FRET) measured by flow cytometry we show that monocytic CD13 co-localized with the Fcγ receptor II/CD32 after Fc receptor ligation by a CD32-specific antibody. FRET was also observed between CD13 and the Fcγ receptor I/CD64, but not with the myeloid marker CD33 representing a member of the sialoadhesin family. Our results imply a novel functional role of CD13 and Fcγ receptors as members of a multimeric receptor complex. Further studies have to be done to elucidate common signaling pathways of these molecules.