Sperm axoneme: A tale of tubulin posttranslation diversity

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Abstract
Two microtubule‐containing structures are assembled during spermiogenesis: a transient manchette and a stable axoneme. Both structures contain microtubules enriched in posttranslationally modified tubulins. Despite the existence of a spectrum of tubulin isotypes postulated by the multi‐tubulin hypothesis, further extended by an elaborated array of posttranslational modifications, it is unknown how this diversity influences microtubule function. There is increasing evidence that different αβ‐tubulin isotypes can affect the structure and function of microtubules. It is also becoming increasingly clear that eukaryotic cells encode other tubulin proteins expressed by the tubulin superfamily: γ, δ ε, ζ η, and FtsZ have been identified so far. Although the role of γ‐tubulin in the nucleation of microtubule assembly is well established, the function of δ‐, ε‐, ζ‐, η‐, and FtsZ‐tubulins is less understood. The members of the tubulin superfamilies found in spermatids include the αβ‐tubulin dimer, in addition to γ‐tubulin in the centrosome, and δ‐tubulin in the perinuclear ring region of the mouse spermatid manchette, the centrosome region, and flagellum. Posttranslational modifications in tubulin isotypes are predominant in the C‐terminus exposed on the outside surface of the microtubule. This target site may influence the interaction of microtubule‐associated proteins, including motor proteins, and therefore determine the functional specificity of tubulin isotypes. It remains to be determined whether other newcomers to the superfamily of tubulins contain sites prone to posttranslational modification. Mol. Reprod. Dev. 62: 1‐3, 2002.