Purification and properties of p103, a novel 103-kDa component of postsynaptic densities

A 103-kDa protein present in membrane cytoskeletal preparations from bovine brain has been identified. We have purified this protein to greater than 95% homogeneity using gel filtration and ion-exchange chromatography. This protein, p103, is an asymmetric dimer in dilute solution and has two major variants that can be distinguished by isoelectric focussing, pI 5.60 and 5.75. Using subcellular fractionation, it is most enriched in postsynaptic densities. Immunolocalization with anti-p103-specific antibodies reveals that it is confined to the dendrites and perikarya; it is apparently absent from spinal cord axons. It coextracts from brain membrane-skeletal preparations with brain spectrin and actin, but in vitro, it does not interact with them.