Non-neuronal 210×103Mr microtubule-associated protein (MAP4) contains a domain homologous to the microtubule-binding domains of neuronal MAP2 and tau

A polyclonal antiserum raised against a HeLa cell microtubule-associated protein of Mr 210,000 (210 kD MAP or MAP4), an abundant non-neuronal MAP, was used to isolate cDNA clones encoding MAP4 from a human fetal brain lambda gt11 cDNA expression library. The largest of these clones, pMAP4.245, contains an insert of 4.1 kb and encodes a 245 kD beta-galactosidase fusion protein. Evidence that pMAP4.245 encodes MAP4 sequences includes immunoabsorption of MAP4 antibodies with the pMAP4.245 fusion protein, as well as identity of protein sequences obtained from HeLa 210 kD MAP4 with amino acid sequences encoded by pMAP4.245. The MAP4.245 cDNA hybridizes to several large (approximately 6-9 kb) transcripts on Northern blots of HeLa cell RNA. DNA sequencing of overlapping MAP4 cDNA clones revealed a long open reading frame containing a C-terminal region with three imperfect 18-amino acid repeats; this region is homologous to a motif present in the microtubule (MT)-binding domain of two prominent neuronal MAPs, MAP2 and tau. The pMAP4.245 sequence also encoded a series of unrelated repeats, located in the MAP's projection domain, N-terminal to the MT-binding domain. MAP4.245 fusion proteins bound to MTs in vitro, while fusion proteins that contained only the projection domain repeats failed to bind specifically to MTs. Thus, the major human non-neuronal MAP resembles two neuronal MAPs in its MT-binding domain, while most of the molecule has sequences, and presumably functions, distinct from those of the neuronal MAPs.