Laser-light scattering diffusion measurements on brain microtubule protein from dogfish (Squalus acanthus) and beef

Laser photon correlation spectroscopy and analytical ultracentrifugation were used to compare the equilibrium properties of disassembled, assembly-competent, brain microtubule protein from dogfish (Squalus acanthus) and beef. Analytical ultracentrifugation confirmed that assembly-competent bovine material at 4 degrees C, purified in the presence of glycerol through 2 cycles of assembly and disassembly, consisted of 6 S (dimer) and 35 S (ring) components, whereas assembly-competent dogfish material prepared in the same way was composed primarily of 6 S protein. By means of photon correlation spectroscopy, z-average diffusion coefficients (D'20, w) of 0.55 (+/- 0.04) x 10(-11) m2 s-1 (beef) and 1.27 (+/- 0.11) x 10(-11) m2 s-1 (dogfish) were measured at 4 degrees C for the twice-cycled assembly competent microtubule protein. Although D'20, w values for dogfish material are significantly higher than those for beef, they depart from the calculated value of 5.0 x 10(-11) m2 s-1 for a pure 6 S (dimer) solution. Microtubule accessory proteins were detectable on sodium dodecyl sulphate/polyacrylamide gels of dogfish microtubule protein. By fitting photocount autocorrelation functions to a 2-particle (dimer-ring) model it was estimated that the presence in a solution of assembly-competent dogfish microtubule protein of less than 1 ring per 250 dimers could account for the measured z-average diffusion coefficient. The temperature sensitivity of D'20, w showed that dogfish brain microtubule protein polymerized at lower temperatures than samples prepared by the same methods from beef brain.