FIG. 1.

PPP2R2B structure and transcription expression. Exons are not to scale. The length of each intron is indicated in kb. Total gene length is approximately 493 kb. Alternate transcripts and their encoded proteins are depicted. Shaded regions of transcripts indicate open reading frames. The predominant transcript begins at exon 7 and encodes Bβ1. The predominant initiation site for transcripts beginning with exon 7 appears to be 3' to the repeat, with much less frequently used start sites 5' to the repeat and within the repeat. Exon 7 includes a multiply spliced internal intron (IVS 7b/7c) and a MED-1 (multiple start-site element downstream 1) sequence (GCTCCC)¹³ 65 bp 3' to the CAG repeat.¹⁰ Intron 7b/7c has two alternate donor sites (28 and 141 bp downstream from the repeat) spliced to a common acceptor site (127 bp upstream from the Bβ1 coding region). Use of the second donor site yields a transcript predicted to encode an alternate N-terminus in which the repeat encodes polyserine. Multiple combinations of upstream exons lead to protein variants with divergent N-termini; Bβ5-7 remain hypothetical. Edited and reprinted with permission from Elsevier.¹²