Abstract
A gene that directs the programmed synthesis of flagellar β-tubulin during the rapid differentiation of Naegleria gruberi from amoebae to flagellates has been cloned and sequenced. The intronless gene is one of 8 to 10 similar but non-identical genes that are dispersed in the genome. β-Tubulin mRNA homologous to this gene family is expressed transiently during differentiation, and has not been detected in amoebae. The encoded β-tubulin is strongly conserved, with features that closely resemble the β-tubulins of diverse organisms, especially organisms that, like Naegleria, use tubulin to assemble flagellar axonemes.
In most sequenced α-tubulins, the encoded carboxy-terminal amino acid is tyrosine, which undergoes post-translational removal and readdition, conserved processes of unknown function. In N. gruberi, unusually, the terminus of α-tubulin is encoded as glutamine while that of β-tubulin is tyrosine. The presence of these divergent termini on subunits of a conserved tubulin provoked us to re-examine aromatic amino acids at the termini of α- and β-tubulins. Although evolution has tinkered extensively with the carboxyterminal domains of tubulin subunits, we find an unexpected conservation. In every organism or cell type for which both tubulin subunits have been sequenced, except the ciliate Stylonychia lemnae, at least one tubulin subunit of some or all tubullin heterodimers terminates in an aromatic amino acid, either tyrosine or phenylalanine. This remarkable conservation of carboxy-terminal aromatic amino acids suggests that these residues serve some crucial function.