Fig. 3. Multimerization is a specific property of the PSD-95 palmitoylation motif. COS cells were transfected with plasmids encoding plasmids as indicated. Cell lysates were immunoprecipitated with an antibody to GFP, FLAG or the PDZ domains of PSD-95 as indicated and western blotting was used to monitor co-immunoprecipitation. (A) Mutating palmitoylated cysteine residues 3 and 5 of PSD-95 blocks co-immunoprecipitation. (B) Mutating leucine 4 of PSD-95 to serine (which blocks palmitoylation) disrupts co-immunoprecipitation. (C) Treating PSD-95-transfected COS cells for 4 hours with 20 µM 2-bromopalmitate – but not palmitate – disrupts multimerization mediated by the N-terminus of PSD-95. (D) Differently tagged chimeras of PSD-95 (containing just the palmitoylated N-terminal ten amino acids of GAP-43 fused to PSD-95 lacking a palmitoylation motif) do not oligomerize.