Characterization of the neuroimmune protein F5: localization to the dendrites and perikarya of mature neurons and the basal aspect of choroid plexus epithelial cells.

F5 was identified originally as an interleukin-2-regulated gene in L2 cells, a murine helper T-lymphocyte clone. In adult mouse, F5 mRNA was expressed at a modest level in lymphoid tissues, at a high level in mature neurons in the nervous system, but not in other tissues. Although the F5 sequence is highly conserved over evolution, the function of the F5 protein is unknown. In the present studies, the putative F5 protein-coding region was translated in vitro using a reticulocyte lysate system and in Escherichia coli, yielding a protein with the predicted molecular weight of 42 kDa. Polyclonal rabbit anti-F5 antibody was generated against a synthetic peptide corresponding to the C-terminus of the F5 protein. This antibody specifically recognized recombinant F5 protein. Western blot studies demonstrated a strongly-reactive 42-kDa band and a faint 39-kDa band in extracts of adult mouse brain regions, the levels of which paralleled F5 mRNA expression. Immunoperoxidase studies of adult mouse brain demonstrated F5 immunoreactivity in neuronal perikarya and dendrites but not axons. Neurons expressing the highest levels of F5 protein corresponded to those with the highest levels of F5 mRNA. Choroid plexus epithelial cells also exhibited strong reactivity localized to their basal aspect. These observations suggest that the F5 protein, expression of which appears to be regulated predominantly at the RNA level, may be involved in the maintenance of the functional or anatomic polarity of neurons and choroid plexus epithelial cells.