Comparisons of microtubule behavior between species point to differences that raise questions about the biological importance of dynamic instability. Notothenioid fish, for example, which live in the Southern Ocean at a constant temperature of –1.8°C, have remarkably stable microtubules compared with warm-blooded vertebrates such as the cow. This is an essential modification for notothenioid fish because normal microtubules disassemble completely into αβ-tubulin dimers at 0°C. Measurements on individual microtubules in solutions of pure tubulin show that notothenioid fish microtubules grow at a much slower rate, shrink at a much slower rate, and only rarely switch from growth to shrinkage (catastrophe) or from shrinkage to growth (rescue) (Table 1). A. The amino acid sequences of the α- and β-tubulin subunits from notothenioid fish differ from those of the cow at positions and in ways that might reasonably be expected to stabilize the microtubule, in accord with the data in Table 16–1. Would you expect these changes to strengthen the interactions between the α- and β-tubulin subunits in the αβ-dimer, between adjacent dimers in the protofilament, or between tubulin subunits in adjacent protofilaments? Explain your reasoning. B. Dynamic instability is thought to play a fundamental role in the rapid microtubule rearrangements that occur in cells. How do you suppose cells in these notothenioid fishes manage to alter their microtubule architecture quickly enough to accomplish essential cell functions? Or do you suppose that these cells exist with a stable microtubule cytoskeleton that only slowly rearranges itself? TABLE 16 domestic cow (Problem 16-79) Microtubules -1 Properties of individual microtubules in notothenioid fish and the Growth Shrinkage Catastrophe Rescue frequency min-1 frequency rate (um/min) (um/min) rate (min 0.008 0.52 Notothenioid fish 0.27 0.8 0.0004 Domestic cow 2.18 61.2 Multiple individual microtubules were observed by video microscopy near the body temperature for each species: 5°C for fish and 37°C for cow. Average growth rates were calculated for growing microtubules, and average shrinkage rates were calculated for shrinking microtubules Changes from growth to shrinkage (catastrophe) and from shrinkage to growth (rescue) were averaged over the observation period and expressed as frequency of events per minute. TABLE 16 domestic cow (Problem 16-79) Microtubules -1 Properties of individual microtubules in notothenioid fish and the Growth Shrinkage Catastrophe Rescue frequency min-1 frequency rate (um/min) (um/min) rate (min 0.008 0.52 Notothenioid fish 0.27 0.8 0.0004 Domestic cow 2.18 61.2 Multiple individual microtubules were observed by video microscopy near the body temperature for each species: 5°C for fish and 37°C for cow. Average growth rates were calculated for growing microtubules, and average shrinkage rates were calculated for shrinking microtubules Changes from growth to shrinkage (catastrophe) and from shrinkage to growth (rescue) were averaged over the observation period and expressed as frequency of events per minute.