Mcl-1 is an anti-apoptotic member of the Bcl-2 family of proteins. We have expressed full length and mutated GFP:Mcl-1 fusion proteins to define structural motifs that control protein localisation and stability, When expressed in U-937 cells, full length Mcl-1 locates primarily within mitochondria and its half-life mas approximately 3 h, which was identical to the native, endogenously expressed protein. When the terminal 20 amino acids from the C-terminus of the protein were detected, the protein mas diffused in the cytoplasm, but its stability was unaffected. This confirms that this region is responsible for efficient targeting to mitochondria, Surprisingly, deletion of 104 amino acids (residues 79-183) that contain putative PEST sequences and other stability regulating motifs, did not affect protein stability. (C) 2000 Federation of European Biochemical Societies. Published by Elsevier Science B.V. All rights reserved.