FIG. 1.

PTB is a component of myoblast protein complexes assembled onto BP and PY. (A) Schematic representation of the two mutually exclusive exons showing the regulatory elements upstream of exon 6B. PmlI and SexAI are restriction enzymes used to construct the plasmid pPmac SexAI. The sequences of the RNAs used for affinity purification of protein complexes are shown below. The A in bold type indicates the branch point upstream of exon 6B (22). (B) Two-dimensional gel electrophoresis of proteins from QM7 myoblast nuclear extracts assembled onto BP, PY, and 3′6B RNAs. Proteins were revealed by silver staining. The insets shown to the right of the BP and PY RNAs represent parts of the Western blot analysis performed on a two-dimensional gel with polyclonal antibodies against PTB. (C) Western blot analysis with anti-PTB of protein complexes assembled on wild-type or mutant RNAs. BPmtPTB and PYmtPTB are mutant BP and PY RNAs, respectively, in which putative PTB binding sites have been mutated (Fig. 1A). Odd and even lanes correspond to QM7 nuclear extracts from myoblasts (mb) and myotubes (mt) assembled onto RNAs, respectively.