Divergent neural β tubulin from the antarctic fish notothenia coriiceps neglecta: Potential sequence contributions to cold adaptation of microtubule assembly

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Abstract
The cytoplasmic microtubules of the cold‐adapted Antarctic fishes, unlike those of homeotherms and temperate poikilotherms, assemble and function at body temperatures in the range −1.8 to +2°C. To determine whether alterations to the primary sequence of β tubulin may contribute to enhancement of microtubule assembly at cold temperatures, we have cloned and sequenced a 1.8‐kilobase neural β‐chain cDNA, Ncnβ1, from an Antarctic rockcod, Notothenia coriiceps neglecta. Based on nucleotide sequence homology, Ncnβ1 probably corresponds to a class‐II β‐tubulin gene. The 446‐residue β chain encoded by Ncnβ1 is closely related (sequence homology ∼95%) both to the neural class‐I/II isotypes and to the neural/testicular class‐IV variants of higher vertebrates, but the sequence of its carboxy‐terminal isotype‐defining region (residues 431–446) has diverged markedly (≥ 25% change relative to the I/II/IV referents). Furthermore, the NcnβsZ1 polypeptide contains six unique amino‐acid substitutions (five conservative, one nonconservative) not found in other vertebrate brain isotypes, and the carboxyterminal region possesses a unique tyrosine inserted at position 442. We conclude that Ncnβ1 encodes a class‐II β tubulin that contains sequence modifications, located largely in its interdimer contact domain, that may contribute to cold adaptation of microtubule assembly.