DIVERGENT NEURAL BETA-TUBULIN FROM THE ANTARCTIC FISH NOTOTHENIA-CORIICEPS-NEGLECTA - POTENTIAL SEQUENCE CONTRIBUTIONS TO COLD ADAPTATION OF MICROTUBULE ASSEMBLY

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-degrees-C. To determine whether alterations to the primary sequence of beta tubulin may contribute to enhancement of microtubule assembly at cold temperatures, we have cloned and sequenced a 1.8-kilobase neural beta-chain cDNA, Ncnbeta1, from an Antarctic rockcod, Notothenia coriiceps neglecta. Based on nucleotide sequence homology, Ncnbeta1 probably corresponds to a class-II beta-tubulin gene. The 446-residue beta chain encoded by Ncnbeta1 is closely related (sequence homology approximately 95% 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 (greater-than-or-equal-to 25% change relative to the I/II/IV referents). Furthermore, the Ncnbeta1 polypeptide contains six unique amino-acid substitutions (five conservative, one nonconservative) not found in other vertebrate brain isotypes, and the carboxy-terminal region possesses a unique tyrosine inserted at position 442. We conclude that Ncnbetal encodes a class-II beta tubulin that contains sequence modifications, located largely in its interdimer contact domain, that may contribute to cold adaptation of microtubule assembly.