Regulation of alternative splicing by PTB and associated factors.
PTB (polypyrimidine tract-binding protein) is a repressive regulator of alternative splicing. We have investigated the role of PTB in three model alternative splicing systems. In the alpha-actinin gene, PTB represses the SM (smooth muscle) exon by binding to key sites in the polypyrimidine tract. Repressive binding to these sites is assisted by co-operative binding to additional downstream sites. SM exon splicing can be activated by CELF proteins, which also bind co-operatively to interspersed sites and displace PTB from the pyrimidine tract. Exon 11 of PTB pre-mRNA is repressed by PTB in an autoregulatory feedback loop. Exon 11-skipped RNA gets degraded through nonsense-mediated decay. Less than 1% of steady-state PTB mRNA is represented by this isoform, but inhibition of nonsense-mediated decay by RNA interference against Upf1 shows that at least 20% of PTB RNA is consumed by this pathway. This represents a widespread but under-appreciated role of alternative splicing in the quantitative regulation of gene expression, an important addition to its role as a generator of protein isoform diversity. Repression of alpha-tropomyosin exon 3 is an exceptional example of PTB regulation, because repression only occurs at high levels in SM cells, despite the fact that PTB is widely expressed. In this case, a PTB-interacting cofactor, raver1, appears to play an important role. By the use of 'tethering' assays, we have identified discrete domains within both PTB and raver1 that mediate their repressive activities on this splicing event.