Fibroblast growth factor-1 interacts with the glucose-regulated protein GRP75/mortalin

Fibroblast growth factor-1 (FGF-1), which lacks a signal peptide and is int racellularly localized as a result of endogenous expression or endocytosis, is thought to be involved in regulating cell growth and differentiation. I n the study reported here, we purified proteins that bind intracellular FGF -1. Affinity adsorption was used to purify FGF-1-binding proteins from rat L6 cells expressing FGF-1. One of the isolated proteins was identified as t he glucose-regulated protein GRP75/mortalin/PBP-74/ mthsp70, a member of th e hsp70 family of heat-shock proteins known to be involved in regulating gl ucose responses, antigen processing and cell mortality. The interaction of FGF-1 and GRP75/mortalin in vivo was confirmed by co-immuno-precipitation, immunohistochemical co-localization in Rat-1 fibroblasts and by using the y east two-hybrid system. Moreover, a binding assay in vitro with the use of recombinant FGF-1 and mortalin demonstrated a direct physical interaction b etween the two proteins. These results reveal that GRP75/mortalin is an int racellular FGF-1-binding protein in cells and suggest that GRP75/mortalin i s involved in the trafficking of and/or signalling by FGF-1.