146
H.W. Detrich
decrease in structural rigidity of the relevant domains. In this context, class-II
NcnJ31 tubulin contains two Ser~Gly substitutions, two Ala~Ser
replacements, and one Val~Ala substitution (Table 2), all of which would
likely increase the flexibility of its amino-terminal, interdimer contact
domain [21,22].
Finally, decreased electrostatic repulsion between tubulin dimers mediated
by reduced posttranslational carboxy-terminal polyglutamylation remains a
viable contributor to cold adaptation of microtubule assembly. Due to
shielding of carboxylate sidechains by counterions and electrostricted water
molecules, its effects may be limited, yet synergistic with the two other
strategies described above.
One key issue that remains is reconciliation of the three categories of
proposed structural adaptations with the strongly entropic control of the
polymerization reaction of Antarctic fish tubulin. The overall energetics of
microtubule elongation must result from summation of the individual
contributions made by all the chemical transformations that accompany
addition of a tubulin dimer to a microtubule. Both the conformational-change
and the electrostatic-repulsion mechanisms described above would be
anticipated to decrease the systemic entropy change for assembly of
Antarctic fish tubulin. Thus, some other structural change, be it an increase
of hydrophobic interactions at dimer-dimer contact sites or another as yet
unrecognized, must enhance the entropic character of the polymerization of
Antarctic fish tubulin relative to the assembly of tubulins from mesotherms.
Cold Adaptation of Microtubule Motors
With respect to microtubule motor proteins of Antarctic fish, our goal is to
determine the molecular adaptations that have evolved to maintain efficient
mechanochemical function at low temperatures. As a first step, we have
evaluated the temperature dependence of the ATPase activities of N
coriiceps inner and outer arm dyne ins at temperatures from 0 to 37 DC [23].
Figure 3 shows that the basal, nonmicrotubule-stimulated ATPase activities
of the fish dyneins, although modest in absolute terms, significantly exceed
that of Tetrahymena outer arm dynein at temperatures ~ 15 DC (at 0 DC, 0.069
and 0.038 Ilmol P/minlmg for inner and outer arm dyneins from N coriiceps
vs. 0.0091lmol P/minlmg for the Tetrahymena enzyme). The inner and outer
arms attain their maximal activities (~O.1llmol P/minlmg) at 9 and 19 DC,
respectively. Above these temperatures, the ATPase activities of the fish
dyneins decline substantially (to 0.050 and 0 Ilmol P/minlmg for inner and
outer arms, respectively, at 37 DC), whereas Tetrahymena dynein activity
continues to increase (e.g., 0.43 Ilmol P/minlmg at 37 DC). The most
H.W. Detrich
decrease in structural rigidity of the relevant domains. In this context, class-II
NcnJ31 tubulin contains two Ser~Gly substitutions, two Ala~Ser
replacements, and one Val~Ala substitution (Table 2), all of which would
likely increase the flexibility of its amino-terminal, interdimer contact
domain [21,22].
Finally, decreased electrostatic repulsion between tubulin dimers mediated
by reduced posttranslational carboxy-terminal polyglutamylation remains a
viable contributor to cold adaptation of microtubule assembly. Due to
shielding of carboxylate sidechains by counterions and electrostricted water
molecules, its effects may be limited, yet synergistic with the two other
strategies described above.
One key issue that remains is reconciliation of the three categories of
proposed structural adaptations with the strongly entropic control of the
polymerization reaction of Antarctic fish tubulin. The overall energetics of
microtubule elongation must result from summation of the individual
contributions made by all the chemical transformations that accompany
addition of a tubulin dimer to a microtubule. Both the conformational-change
and the electrostatic-repulsion mechanisms described above would be
anticipated to decrease the systemic entropy change for assembly of
Antarctic fish tubulin. Thus, some other structural change, be it an increase
of hydrophobic interactions at dimer-dimer contact sites or another as yet
unrecognized, must enhance the entropic character of the polymerization of
Antarctic fish tubulin relative to the assembly of tubulins from mesotherms.
Cold Adaptation of Microtubule Motors
With respect to microtubule motor proteins of Antarctic fish, our goal is to
determine the molecular adaptations that have evolved to maintain efficient
mechanochemical function at low temperatures. As a first step, we have
evaluated the temperature dependence of the ATPase activities of N
coriiceps inner and outer arm dyne ins at temperatures from 0 to 37 DC [23].
Figure 3 shows that the basal, nonmicrotubule-stimulated ATPase activities
of the fish dyneins, although modest in absolute terms, significantly exceed
that of Tetrahymena outer arm dynein at temperatures ~ 15 DC (at 0 DC, 0.069
and 0.038 Ilmol P/minlmg for inner and outer arm dyneins from N coriiceps
vs. 0.0091lmol P/minlmg for the Tetrahymena enzyme). The inner and outer
arms attain their maximal activities (~O.1llmol P/minlmg) at 9 and 19 DC,
respectively. Above these temperatures, the ATPase activities of the fish
dyneins decline substantially (to 0.050 and 0 Ilmol P/minlmg for inner and
outer arms, respectively, at 37 DC), whereas Tetrahymena dynein activity
continues to increase (e.g., 0.43 Ilmol P/minlmg at 37 DC). The most