Localization of 350K molecular weight and related proteins in both the cytoskeleton and nuclear flecks that increase during G1 phase.

Monoclonal and polyclonal antibodies were raised against the highest molecular weight microtubule-associated protein (MAP-1) isolated from brain. Immunoblotting with the antibodies revealed the presence of cross-reactive protein of 350K or less on whole cells, isolated nuclei and cellular microtubules. Two-dimensional peptide maps showed substantial homology of immunoprecipitated cellular proteins of 350K, 80K and 51K with a 25K fragment of brain MAP-1. On antibody staining, immunofluorescence was seen on a cytoplasmic network, the mitotic spindle, the centrosome, and intranuclear flecks. The antibody causing immunofluorescence in all these sites was absorbed most effectively with slices of blotted membrane which contained the 350K protein. These results suggest that the cross-reactive molecules in diverse sites belong to the family of the 350K protein. The number of nuclear flecks and the amount of bound radioactivity of 125I-antibody almost doubled during G1 phase.