Growth inhibitory factor (GIF), renamed as metallothionein-3, is a 68-amino acids brain specific member of the metallothionein family. GIF plays comprehensively physiologic roles and is deemed to be implicated in Alzheimer's disease. When supplemented with Alzheimer's brain extracts, GIF demonstrates its in vitro growth inhibitory activity on cultured neuron; but the underlying molecular mechanism remains unknown. To probe this mystery, the yeast two-hybrid system was employed to screen a human brain cDNA library for GIF-interacting proteins. From 4.1×10⁶ transformants of library screening, a C-terminal fragment of small G-protein Rab3a was obtained, harboring 87 amino acids that interacted with GIF in yeast. The full-length Rab3a was amplified by polymerase chain reaction (PCR) using human total placenta cDNA as template and concomitantly cloned. In yeast two-hybrid test, full-length Rab3a also interacted with GIF. Through coimmunoprecipitation and Western blotting experiments, interaction between GIF and Rab3a was confirmed in mammalian cell. Furthermore, the result indicated that Rab3a interacted with GIF in a GTP-binding form.