Four repeat high-mol-wt MAP2 forms in rat dorsal root ganglia.

The high-mol-wt forms of brain microtubule-associated protein 2 (MAP2a/b) segregate within the dendrites during neuronal differentiation, whereas a low-mol-wt variant, MAP2c, is distributed within all the neuronal domains. Both MAP2b and MAP2c contain three tubulin binding repeats, whereas another low-mol-wt form, MAP2d, contains four repeats. Since high-mol-wt MAP2 species with four repeats have been cloned so far only from the sensory ND cell line, we have studied in this work the high-mol-wt forms expressed by dorsal root ganglia (DRG). Different clones obtained from PCR amplification products of portions of the C-terminal and of the 3' end of the middle MAP2b domains contained either three or four tubulin binding repeats at adult stages and only three postnatally. In adulthood, two exons located at the 3' end of the MAP2b middle domain were missing in several clones: exon 10 within clones with three or four repeats, exon 11 only within those containing four repeats. Several other clones obtained from PCR amplification products of portions of the N-terminal and of the 5' end of the middle MAP2b domains revealed exons 7A and 8. In contrast, Northern analysis revealed exon 8 but not exon 7A, which is probably expressed in trace amounts in the DRG. In this article, we have identified for the first time high-mol-wt MAP2 transcripts containing four tubulin binding repeats that seem to be expressed only by the DRG and also differing from brain MAP2b by a number of other exons.