  In eukaryotic cells, KDEL receptors (KDELRs) facilitate the retrieval of
endoplasmic reticulum (ER) luminal proteins from the Golgi compartment back to
the ER. Apart from the well-documented retention function, recent findings
reveal that the cellular KDELRs have more complex roles, e.g. in cell
signalling, protein secretion, cell adhesion and tumorigenesis. Furthermore,
several studies suggest that a sub-population of KDELRs is located at the cell
surface, where they could form and internalize KDELR/cargo clusters after
K/HDEL-ligand binding. However, so far it has been unclear whether there are
cell-type- or species-specific differences in KDELR clustering. By comparing
ligand-induced KDELR clustering in different mouse and human cell lines via
live cell imaging, we show that macrophage cell lines from both species do not
develop any clusters. Using RT-qPCR experiments and numerical analysis, we
address the role of KDELR expression as well as endocytosis and exocytosis
rates on the receptor clustering at the plasma membrane and discuss how the
efficiency of directed transport to preferred docking sites on the membrane
influences the exponent of the power-law distribution of the cluster size.
