Isolated microtubule proteins from the cold-adapted fish, Atlantic cod (Gadus morhua), assemble at temperatures between 8 and 30 degrees C, while avian and mammalian microtubules normally do not assemble at temperatures below 20 degrees C. Tubulin, the main component in microtubules, is expressed as many isotypes, Microtubules with different isotype composition have been shown to have different dynamic properties in vitro. Our hypothesis was that cold-tolerance of microtubules is caused by tubulin isotypes that differ in the primary sequence compared to mammalian tubulins, Here we show that transfection of human HepG2 cells with cod beta-tubulin induced cold-adaptation of the endogenous microtubules, Incorporation of one single tubulin isotype can induce cold-tolerance to cold-intolerant microtubules, Three cod beta-tubulin isotypes were tested and two of these (beta 1 and beta 2) transferred cold-tolerance to HepG2 microtubules, thus not all cod beta-tubulins were able to confer cold-stability. 8 2000 Academic Press.