FIG. 9.

Models to explain how nonneuronal and neuron-specific alternative RNA processing of the human calcitonin/CGRP pre-mRNA are achieved. In nonneuronal cells, a number of trans-acting factors promote exon 4 inclusion at both ends of this exon. In neuronal cells, Fox proteins bind to UGCAUG elements and interfere with binding of splicing factors at the 3′ splice site, whereas Hu proteins antagonize the activity of TIAR protein leading to decreased exon 4 inclusion. The dashed line indicates a hypothesized role of Fox proteins in interfering with SF1 binding at the branchpoint sequence.