MICROTUBULE ASSEMBLY IN COLD-ADAPTED ORGANISMS - FUNCTIONAL-PROPERTIES AND STRUCTURAL ADAPTATIONS OF TUBULINS FROM ANTARCTIC FISHES

Fishes native to the coastal waters of the Antarctic have adapted to h abitat and body temperatures in the range -1.8 to +2 degrees C. Their cytoplasmic microtubules, unlike those of mammals and temperate poikil otherms, have evolved to assemble efficiently at these low temperature s. To learn about the underlying molecular adaptations, my laboratory is studying microtubule proteins [tubulin alpha beta dimers and microt ubule-associated proteins (MAPs)] and tubulin genes from several Antar ctic fishes, including the rockcods Notothenia coriicepts and Gobionot othen gibberifrons. We find that the assembly-enhancing adaptations of the fish microtubule proteins are intrinsic to the tubulin subunits t hemselves. Furthermore, microtubule formation by Antarctic fish tubuli ns is strongly entropy driven, due in part-to an increased reliance, r elative to tubulins from other species, on hydrophobic interactions. B ased on analyses of tubulin polypeptides and cDNAs, we suggest that th e structural adaptations of Antarctic fish tubulins most likely involv e alterations in the primary sequences of tubulin isotypes. With respe ct to neural beta tubulins from other vertebrates, for example, the cl ass II beta-tubulin isotype of N. coriiceps brain contains seven uniqu e amino acid substitutions and one novel insertion in its 446-residue primary sequence. Most of these changes are located in a structural do main that forms contacts between tubulin dimers during microtubule ass embly and would be expected to enhance polypeptide flexibility, thereb y facilitating addition of tubulin to microtubule ends. The acidic car boxy-terminal tails of the alpha and beta tubulins, by contrast, appea r not to be sites of cold adaptation of polymerization. We have also f ound that brain and egg tubulins from Antarctic fishes differ striking ly in their polymerization efficiencies, which demonstrates, in agreem ent with the multitubulin hypothesis, that tissue-specific tubulin iso forms can possess distinct functional properties. Thus, study of micro tubule proteins from organisms, such as the Antarctic fishes, that hav e adapted to extreme thermal regimes should contribute significantly t o an understanding of the quaternary interactions that control microtu bule assembly in all eukaryotes. (C) 1997 Elsevier Science Inc.