TIP47 Is Not a Component of Lipid Droplets

Abstract

TIP47 functions in the delivery of mannose 6-phosphate receptors from endosomes to the trans-Golgi network both in vitro and in vivo. It binds directly and very specifically to the cytoplasmic domains of both the cation-independent and cation-dependent mannose 6-phosphate receptors. TIP47 is 43% identical to a lipid droplet-associated protein named adipophilin; much of the identity resides near the N termini of these proteins. It was recently reported in this journal, in a study using antiserum from this laboratory, that TIP47 is a constituent of lipid droplets (Wolins, N. E., Rubin, B., and Brasaemle, D. L. (2001) J. Biol. Chem. 276, 5101-5108). We show here that the findings of Wolins et al. were likely due to either a cross-reactive, unidentified protein in HeLa cells that is recognized by our antiserum and/or the fact that our serum also cross-reacts with the adipophilin protein itself, shown directly by expression of adipophilin in Escherichia coli. Using antibodies specific for residues 152-434 of TIP47, we show that TIP47 is not a constituent of lipid droplets.