Cold adaptation of microtubule assembly and dynamics - Structural interpretation of primary sequence changes present in the alpha- and beta-tubulins of antarctic fishes

The microtubules of Antarctic fishes, unlike those of homeotherms, assemble at very low temperatures (-1.8 degreesC). The adaptations that enhance ass embly of these microtubules are intrinsic to the tubulin dimer and reduce i ts critical concentration for polymerization at 0 degreesC to similar to0.9 mg/ml (Williams, R, C., Jr., Correia, J. J., and DeVries, A. L. (1985) Bio chemistry 24, 2790-2798). Here we demonstrate that microtubules formed by p ure brain tubulins of Antarctic fishes exhibit slow dynamics at both low (5 degreesC) and high (25 degreesC) temperatures; the rates of polymer growth and shortening and the frequencies of interconversion between these states are small relative to those observed for mammalian microtubules (37 degree sC). To investigate the contribution of tubulin primary sequence variation to the functional properties of the microtubules of Antarctic fishes, we ha ve sequenced brain cDNAs that encode 9 alpha -tubulins and 4 beta -tubulins from the yellowbelly rockcod Notothenia coriiceps and 4 alpha -tubulins an d 2 beta -tubulins from the ocellated icefish Chionodraco rastrospinosus. T he tubulins of these fishes mere found to contain small sets of unique or r are residue substitutions that mapped to the lateral, interprotofilament su rfaces or to the interiors of the alpha- and beta -polypeptides; longitudin al interaction surfaces are not altered in the fish tubulins. Four changes (A278T and S287T in alpha; S280G and A285S in beta) were present in the S7- H9 interprotofilament ''M'' loops of some monomers and would be expected to increase the flexibility of these regions. A fifth lateral substitution sp ecific to the alpha -chain (M302L or M302F) may increase the hydrophobicity of the interprotofilament interaction. Two hydrophobic substitutions (alph a :S187A in helix H5 and beta :Y202F in sheet S6) may act to stabilize the monomers in conformations favorable to polymerization. We propose that cold adaptation of microtubule assembly in Antarctic fishes has occurred in par t by evolutionary restructuring of the lateral surfaces and the cores of th e tubulin monomers.