Tubulin is the material of which microtubules in cells are made. Specifically, alpha tubulin and beta tubulin are the specific proteins, which form a dimer of which the microtubule is made.

As to the function of microtubules, Lehninger's biochemistry text ("Principles of Biochemistry", 1982, page 30) states "They participate in the action of the mitotic spindle during cell division, and they also serve as the motile units in eukaryotic cilia and flagella."

37 kD is of course 37,000 Daltons. One Dalton = either 1.66024 x 10e-24 or 1.65979 x10e-24, depending on whether you're talking chemistry or physics (according to CRC Handbook of Chemistry and Physics, 60th Ed., page F-312).

Basic factoid contributed by alert reader Sergey Levin. Further information by alert reader Tom Matches, who thinks that he told me about this 37 factoid before, but I think he's confused. (Note that our thoughts are not mutually exclusive.)