Je. Dominguez et al., A PROTEIN RELATED TO BRAIN MICROTUBULE-ASSOCIATED PROTEIN MAP1B IS A COMPONENT OF THE MAMMALIAN CENTROSOME, Journal of Cell Science, 107, 1994, pp. 601-611
The centrosome is the main microtubule organizing center of mammalian
cells. Structurally, it is composed of a pair of centrioles surrounded
by a fibro-granular material (the pericentriolar material) from which
microtubules are nucleated. However, the nature of centrosomal molecu
les involved in microtubule nucleation is still obscure. Since brain m
icrotubule-associated proteins (MAPs) lower the critical tubulin conce
ntration required for microtubule nucleation in tubulin solution in vi
tro, we have examined their possible association with centrosomes. By
immunofluorescence, monoclonal and polyclonal antibodies raised agains
t MAP1B stain the centrosome in cultured cells as well as purified cen
trosomes, whereas antibodies raised against MAP2 give a completely neg
ative reaction. The MAP1B-related antigen is localized to the pericent
riolar material as revealed by immunoelectron microscopy. In preparati
ons of purified centrosomes analyzed on polyacrylamide gels, a protein
that migrates as brain MAP1B is present. After blotting on nitrocellu
lose, it is decorated by anti-MAP1B antibodies and the amino acid sequ
ence of proteolytic fragments of this protein is similar to brain MAP1
B. Moreover, brain MAP1B and its centrosomal counterpart share the sam
e phosphorylation features and have similar peptide maps. These data s
trongly suggest that a protein homologue to MAP1B is present in centro
somes and it is a good candidate for being involved in the nucleating
activity of the pericentriolar material.