Figure 3.

The anti-P target p331 is an integral membrane protein present in rat brain synaptosomes. (A) Detection of p331 in rat brain synaptosomes. Membrane fractions prepared from rat brain synaptosomes were analyzed by immunoblot (IB) with control sera from healthy individuals (lanes 1 and 2), anti-P (−) from two SLE patients (lanes 3 and 4), and anti-P (+) from six SLE patients (lanes 5–10), including one with psychosis (lane 10). All P (+) serum detected p331, providing further evidence of its expression in brain neurons. (B) p331 is an integral membrane protein and the main α-hP₁₁ target in synaptosomes. Immunoblot of synaptosomes (Syn) with affinity-purified α-hP₁₁ antibodies shows p331 as the main target (lane 1). The lower bands are degradation products that vary in different experiments. Sodium carbonate extraction of synaptosomal membranes shows that p331 distributes exclusively to the pellet (P; lane 2), similar to the integral membrane protein dopamine receptor (D2DR), whereas peripherally associated clathrin distributes to the supernatant (SN; lane 3).