Molecular Adaptation of Microtubules and Microtubule Motors from Antarctic Fish
143
polymerization thermodynamics of tubulins is conservation of the critical
concentration for microtubule assembly. Measured in vitro at physiologically
relevant temperatures, critical concentrations for tubulins from cold-adapted
Antarctic fish, from several temperate invertebrates (sea urchins, clams), and
from mammals cluster in the range 0.4-2 mg/ml (reviewed by [3]). Thus,
organisms from disparate thermal regimes are able to form microtubules
efficiently within the normal limits of their body temperatures.
Structural Determinants of the Polymerization of Antarctic
Fish Tubulins
The results presented above indicate that Antarctic fish have evolved brain
tubulins that have an enhanced capacity to polymerize at low temperatures.
Our challenge now is to explain these functional properties in terms of
molecular changes to the tubulin subunits.
Strategies to Enhance Protein Polymerization Reactions at Low
Temperatures
Functional adaptation of Antarctic fish tubulins might result from two
general evolutionary strategies: 1) increased reliance on polymer-stabilizing
interactions; and 2) reduction in polymer-destabilizing interactions.
Constituting the first category might be primary sequence changes that
increase the hydrophobicity and/or flexibility oftubulin domains involved in
interdimer contact. In the latter we place reduction of electrostatic repulsion
between the exceptionally acidic tubulin dimers. This most likely would be
mediated by differential posttranslational polyglutamylation of the tubulin
chains. We have examined these possibilities, which are not mutually
exclusive, by comparative analysis of the primary sequences and
posttranslational modifications of tubulins from Antarctic fish and from
temperate mesophiles.
Primary Sequences and Posttranslational Modifications of
Antarctic Fish Tubulins
To investigate the potential contribution of primary sequence variation to
cold adaptation of microtubule assembly, my laboratory has initiated the
cloning and sequence analysis of u- and p-tubulin cDNAs from N coriiceps.
As an example, we isolated and sequenced a 1.8-kilobase (kb) neural ptubulin cDNA, Ncnpi [11]. The p tubulin encoded by Ncnpl is most closely
related to the neural p chains (classes I-IV) of other vertebrates [12-14] and
143
polymerization thermodynamics of tubulins is conservation of the critical
concentration for microtubule assembly. Measured in vitro at physiologically
relevant temperatures, critical concentrations for tubulins from cold-adapted
Antarctic fish, from several temperate invertebrates (sea urchins, clams), and
from mammals cluster in the range 0.4-2 mg/ml (reviewed by [3]). Thus,
organisms from disparate thermal regimes are able to form microtubules
efficiently within the normal limits of their body temperatures.
Structural Determinants of the Polymerization of Antarctic
Fish Tubulins
The results presented above indicate that Antarctic fish have evolved brain
tubulins that have an enhanced capacity to polymerize at low temperatures.
Our challenge now is to explain these functional properties in terms of
molecular changes to the tubulin subunits.
Strategies to Enhance Protein Polymerization Reactions at Low
Temperatures
Functional adaptation of Antarctic fish tubulins might result from two
general evolutionary strategies: 1) increased reliance on polymer-stabilizing
interactions; and 2) reduction in polymer-destabilizing interactions.
Constituting the first category might be primary sequence changes that
increase the hydrophobicity and/or flexibility oftubulin domains involved in
interdimer contact. In the latter we place reduction of electrostatic repulsion
between the exceptionally acidic tubulin dimers. This most likely would be
mediated by differential posttranslational polyglutamylation of the tubulin
chains. We have examined these possibilities, which are not mutually
exclusive, by comparative analysis of the primary sequences and
posttranslational modifications of tubulins from Antarctic fish and from
temperate mesophiles.
Primary Sequences and Posttranslational Modifications of
Antarctic Fish Tubulins
To investigate the potential contribution of primary sequence variation to
cold adaptation of microtubule assembly, my laboratory has initiated the
cloning and sequence analysis of u- and p-tubulin cDNAs from N coriiceps.
As an example, we isolated and sequenced a 1.8-kilobase (kb) neural ptubulin cDNA, Ncnpi [11]. The p tubulin encoded by Ncnpl is most closely
related to the neural p chains (classes I-IV) of other vertebrates [12-14] and