Adaptor Protein Ruk/CIN85 is Associated with a Subset of COPI‐Coated Membranes of the Golgi Complex

Regulator of ubiquitous kinase/Cbl‐interacting protein of 85 kDa (Ruk/CIN85) and CD2‐associated protein/Cas ligand with multiple SH3 domains (CD2AP/CMS) comprise a family of vertebrate adaptor proteins involved in several important cellular processes, including downregulation of activated receptor tyrosine kinases, regulation of cytoskeletal rearrangements, phosphatidylinositol 3‐kinase (PI 3‐kinase) signalling and apoptosis. The role of Ruk/CIN85 as a scaffold protein involved in membrane trafficking processes has been demonstrated in model cell systems. However, intracellular localization of endogenous Ruk/CIN85 has never been comprehensively assessed. We carried out detailed studies of subcellular distribution of Ruk/CIN85 in adherent cultured human cells using antibodies that recognize distinct epitopes of the protein and revealed a punctate immunostaining pattern, common for proteins involved in intracellular trafficking processes. Our data indicate that Ruk/CIN85 is distributed between several different membrane trafficking compartments, but the major pool of Ruk/CIN85 is associated with the Golgi complex, mainly with a subpopulation of COPI‐coated vesicles involved in retrograde endoplasmic reticulum‐Golgi and intra‐Golgi transport. This localization pattern is dependent on the integrity of Golgi complex and intact microtubular network. Only a small pool of Ruk/CIN85 is present in compartments involved in clathrin‐mediated endocytosis and sorting. These results suggest that endogenous Ruk/CIN85 may be involved in regulation of specific membrane trafficking processes.