The splicing determinants of a regulated exon in the axonal MAP tau reside within the exon and in its upstream intron.

Tau is a microtubule-associated protein whose transcript undergoes complex regulated splicing in the mammalian nervous system. Exon 6 of the gene is an alternatively spliced cassette whose expression profile is distinct from that of the other tau regulated exons, implying the utilization of distinct regulatory factors. Previous work had established the use of cryptic splice sites within exon 6 and the influence of flanking exons on the ratio of exon 6 variants. The present work shows that, in addition to the previously identified participants, the ratio of exon 6 isoforms is affected by: (1) suppression of the cryptic sites, (2) deletions of the upstream intron, and (3) the splicing regulators PTB and U2AF, both of which act on the branch point/polypyrimidine tract region. These results strongly suggest that factors binding immediately upstream of exon 6 are involved in regulation of this exon. They also lead to the conclusion that splicing of exon 6 is primarily governed by multiple branch points.