Fluorescent cell division cycle (CDC)48 proteins were Studied in living plant protoplasts. CDC48A and somatic embryogenesis receptor like kinase I (SERK1) were found to co-localize in the endoplasmatic reticulum (ER) and at the plasma membrane (PM), but not in endosomal compartments. Fluorescent lifetime imaging microscopy (FLIM) was used to detect Forster resonance energy transfer (FRET) between CrFP/YFP-tagged CDC48A and SERK I. FRET is indicative of direct protein-protein interaction. CDC48A was found to interact only with SERK I in small areas at the PM, but not in endosomes. These findings confirm and extend our previous findings that CDC48A in plants directly interacts with SERK1. (c) 2006 Elsevier Inc. All rights reserved.