A new soluble brain-specific protein: identification and partial purification

A new soluble 170-kDa protein (BP170) was found to be present exclusively in the brain of all the vertebrates that we studied by Western immunoblotting. It was not detected in peripheral rat tissues, including heart, kidney, liver, spleen, lung, muscle, adrenal, intestinal mucosa, sciatic nerve, or pituitary. In rat brain, its regional distribution was found to be heterogeneous, with its highest concentration in the cerebrum and its lowest in the hypothalamus, and 89% of it was in the post-microsomal fraction. BP170 constitutes at least 0.05% of the total brain cytosol proteins. Its level increases during development, being the lowest at 5 days and the highest at 90 days postnatal. BP170 is a single-chain polypeptide. It could be partially purified by precipitation with polyethylene glycol followed by column chromatography on Q Sepharose. Although BP170 was identified by an antiserum against puromycin-sensitive aminopeptidase (PSA), the two proteins differ in molecular weight, chromatographic properties, regional and subcellular distribution, developmental changes, immunoreactivity, and enzyme activity. Self-incubation or trypsin treatment of the partially purified BP170 generates no PSA activity, indicating that BP170 is not a PSA precursor. Furthermore, BP170 is neither an inhibitor nor an activator of PSA. Our data suggest that BP170 is a novel brain-specific protein not previously described.