Five mouse tubulin isotypes and their regulated expression during development

We describe five mouse tubulin cloned cDNAs, two (M alpha 1 and M alpha 2) that encode alpha-tubulin and three (M beta 2, M beta 4, and M beta 5) that encode beta-tubulin. The sequence of these clones reveals that each represents a distinct gene product. Within the sequence common to the two alpha-tubulin cDNAs, the encoded amino acids are identical, though the 3' noncoding regions are wholly dissimilar. In contrast, the three beta-tubulin cDNAs show considerable carboxy-terminal heterogeneity. Two of the beta-tubulin isotypes defined by the cloned sequences are absolutely conserved between mouse and human, and all three beta-tubulin isotypes are conserved between mouse and rat. This result implies the existence of selective constraints that have maintained sequence identity after species divergence. This conclusion is reinforced by the near identity between a third mouse beta-tubulin isotype and a chicken beta-tubulin isotype. The significance of the interspecies conservation of tubulin isotypes is discussed in relationship to microtubule function. We have used non-cross-hybridizing 3' noncoding region probes from the five cloned mouse tubulin cDNAs to study the developmental expression of each isotype in various mouse tissues. M alpha 1 and M beta 2 are expressed in an approximately coordinate fashion, and their transcripts are most abundant in brain and lung. M alpha 2 and M beta 5 are ubiquitously expressed and to a similar extent in each tissue, with the greatest abundance in spleen, thymus, and immature brain. In contrast, M beta 4 is expressed exclusively in brain. Whereas the expression of the latter isotype increases dramatically during postnatal development, transcripts from all four other tubulin genes decline from maximum levels at or before birth. Tissue-specific development changes in the abundance of tubulin isotype-specific mRNAs are discussed in relationship to organogenesis in the mouse.