Tubulin was isolated from two species of antarctic fish, Pagothenia borchgrevinki and Dissostichus mawsoni, by cycles of temperature-dependent assembly, centrifugation, disassembly, and centrifugation. The preparations were found to consist almost entirely of tubulin and to contain negligibly small amounts of microtubule-associated proteins. This tubulin polymerized to make microtubules of ordinary dimensions. The formed microtubules appear to be in labile equilibrium with free tubulin dimer at all temperatures observed. In a buffer consisting of 0.1 M 1,4-piperazinediethanesulfonic acid, 2 mM dithioerythritol, 1 mM MgS04, 2 mM ethylene glycol bis(β-aminoethyl ether)-N, N, N', N-tetraacetic acid, and 1 mM guanosine 5'-triphosphate, pH 6.9, the tubulin of P. borchgrevinki has a critical concentration for assembly of 0.046 (t0.008) mg/mL at 35 °C and 0.74 (±0.15) mg/mL at the habitat temperature of the fish, -1.8 °C. The tical concentration measured at the lower temperature is quite small relative to the critical concentration formation of mammalian microtubules from pure tubulin at the same temperature, which must be at li st 2 orders of magnitude larger. The antarctic fish microtubules may thus be called "cold stable" by comparison with mammalian microtubules. They do not fully dissociate at temperatures near 0 °C because they are composed of tubulin that assembles more readily at these temperatures than does mammalian tubulin. There is no evidence for the presence of a cold-stabilizing factor in association with the tubulin. These findings suggest that alteration of tubulin may be a means by which some poikilotherms can adapt to a cold environment. A van't Hoff plot of the apparent association constants measured between -1.2 and 35 °C was linear and yielded an apparent ΔH0 of +13.7 kcal/mol and an apparent ΔS° of +74 eu for the addition of a fish tubulin subunit to the end of a growing microtubule.
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