SUZ12 immunoprecipitation followed by PRM of SUZ12 peptides to identify alternatively spliced isoforms of SUZ12 at the protein level.

Alternative splicing of SUZ12 exon 4 is predicted to generate two alternative isoforms differing by the inclusion or exclusion of 69 nucleotides in exon 4, which we named SUZ12-long (SUZ12-L) and SUZ12-short (SUZ12-S) respectively. At the protein level SUZ12 exon 4 encodes 23 amino acids (aa 129–152 in SUZ12-L) that partially overlap with the WD-binding domain 1 (WDB1, 110–145). To detect the existance of SUZ12-S at the protein levels we performed SUZ12 immunoprecipitation coupled with mass spectrometry (IP-MS) in the WT and ∆ex4 clones to identify SUZ12-S unique peptides. We identified a SUZ12-S specific peptide in both WT #2 and ∆ex4 #1. These observations were confirmed by SUZ12 IP followed by Western blot (WB) (size shift).