Microtubule-associated proteins MAP5 and MAP1x: closely related components of the neuronal cytoskeleton with different cytoplasmic distributions in the developing brain.

Monoclonal antibodies were used to explore the relationship between two similarly sized microtubule-associated proteins (MAPs), MAP1x and MAP5. Although the proteins detected by anti-MAP1x and anti-MAP5 co-migrate in SDS-polyacrylamide gels, the patterns of antigenic proteolytic fragments (epitope maps) derived from them were completely different. The results suggest either that MAP1x is more stable than MAP5 or that the MAP1x epitope is situated close to one end of the molecule and gives rise to a very short proteolytic fragment. Immunoprecipitation from brain supernatants with either antibody brought down protein that cross-reacted with the other antibody, indicating that individual molecules bearing both epitopes exist in brain. Peptide maps of the proteins immunoprecipitated with the two antibodies showed that they are closely similar. Despite these similarities, the two antibodies gave different staining patterns on sections of developing rat brain, anti-MAP5 staining both axons and dendrites whereas anti-MAP1x stained only axons. We conclude that the MAP5 and MAP1x molecules are very similar, and possibly identical. The difference in staining patterns with the two antibodies could be because there are two proteins present in brain, one in immature axons bearing both the MAP5 and MAP1x epitopes and another with a wider distribution bearing only the MAP5 epitope. Alternatively, there may be a single protein bearing both epitopes, with the MAP1x epitope being masked in neuronal dendrites and mature axons by covalent modification or inter-molecular binding.