Requirement for the β_I and β_IV tubulin isotypes in mammalian cilia

[Nielsen et al., 2001: Curr Biol 11:529-533], based on studies in Drosophila, have proposed that β tubulin in axonemal microtubules must contain a specific acidic seven amino acid sequence in its carboxyl terminus. In mammals, the two β_IV isotypes (β_IVa and β_IVb) contain that sequence. In order to test the application of this hypothesis to mammals, we have examined the expression of β tubulin isotypes in four different ciliated tissues (trachea, ependyma, uterine tube, and testis) using isotype-specific antibodies and indirect immunofluorescence. We find that β_IV tubulin is present in all ciliated cell types examined, but so is β₁ tubulin. Taken together with recent studies that show that β₁ and β_IV tubulin are both present in the cilia of vestibular hair cells, olfactory neurons, and nasal respiratory epithelial cells, we propose that both β₁ tubulin and β_IV tubulin may be required for axonemal structures in mammals.