The SM19 gene, required for duplication of basal bodies in Paramecium, encodes a novel tubulin, eta-tubulin

The discovery of delta -tubulin, the fourth member of the tubulin superfami ly, in Chiamydomonas [1] has led to the identification in the genomes of ve rtebrates and protozoa of putative delta homologues and of additional tubul ins, epsilon and zeta [2-4]. These discoveries raise questions concerning t he functions of these novel tubulins, their interactions with microtubule a rrays and microtubule-organising centres, and their evolutionary status. Th e sm19-1 mutation of Paramecium specifically inhibits basal body duplicatio n [5] and causes delocalisation of gamma -tubulin, which is also ;required for basal body duplication [6]. We have cloned the SM19 gene by functional complementation and found that it encodes another new member of the tubulin superfamily. SM19p, provisionally called etatubulin (eta -tubulin), shows low sequence identity with the tubulins previously identified in Paramecium , namely, alpha [7], beta [8], gamma [6], delta (this work) and epsilon (P. Dupuis-Williams, personal communication). Phylogenetic analysis indicated that SM19p is not consistently grouped with any phylogenetic entity.