A UNIQUE INTRONIC SPLICING ENHANCER CONTROLS THE INCLUSION OF THE AGRIN-Y EXON

Alternative splicing of the agrin mRNA controls the ability of agrin p rotein to induce the clustering of acetylcholine receptors at the neur omuscular junction. Using a transfectable reporter gene, we show that one agrin alternative exon, the Y exon, is controlled by a regulatory sequence in the downstream intron. Portions of this intronic sequence have the properties of a splicing enhancer that can activate splicing of a heterologous exon when placed in the intron downstream. The regul atory region is complex in structure, containing several different ele ments capable of activating splicing. Individual enhancing elements di ffer in their cell-type specificity, and are not apparently synergisti c, as two elements together induce lower splicing than either does sep arately. Essential nucleotides within these regulatory elements were i dentified by scanning mutagenesis across the active region. Interestin gly, the elements do not appear similar to known intronic splicing enh ancer elements. This Y exon enhancer and its components take part in a n apparent combinatorial system of control where multiple regulatory e lements of varying activity combine to produce a precisely cell-specif ic exon inclusion. As a major contributor to the regulation of the Y e xon, the enhancer ultimately controls the properties of the agrin prot ein.