The mammalian β-tubulin repertoire: Hematopoietic expression of a novel, heterologous β-tubulin isotype

Abstract

We describe the structure of a novel and unusually heterologous β-tubulin isotype (Mβ1) isolated from a mouse bone marrow cDNA library, and a second isotype (Mβ3) isolated from a mouse testis cDNA library. Comparison of Mβ1 and Mβ3 with the completed (Mβ4, Mβ5) or extended (Mβ2) sequence of three previously described β-tubulin isotypes shows that each includes a distinctive carboxy-terminal region, in addition to multiple amino acid substitutions throughout the polypeptide chain. In every case where a mammalian interspecies comparison can be made, both the carboxy-terminal and internal amino acid substitutions that distinguish one isotype from another are absolutely conserved. We conclude that these characteristic differences are important in determining functional distinctions between different kinds of microtubule. The amino acid homologies between Mβ2, Mβ3, Mβ4, and Mβ5 are in the range of 95-97%; however the homology between Mβ1 and all the other isotypes is very much less (78%). The dramatic divergence in Mβ1 is due to multiple changes that occur throughout the polypeptide chain. The overall level of expression of Mβ1 is low, and is restricted to those tissues (bone marrow, spleen, developing liver and lung) that are active in hematopoiesis in the mouse. We predict that the Mβ1 isotype is functionally specialized for assembly into the mammalian marginal band.