DIFFERENTIAL EXPRESSION OF TUBULIN ISOTYPES DURING THE CELL-CYCLE

Microtubules play an essential role in cell division. Little is known about possible variations of total tubulin and tubulin isotype express ion during the cell cycle. We analyzed the total tubulin content, tubu lin polymerization status and tubulin isotype content in resting and d ividing human K562 leukemic cells and human MES-SA sarcoma cells. Alth ough the total cellular tubulin content increases as the cells progres s toward mitosis, the total tubulin/total protein ratio is stable duri ng the cell cycle. Reverse transcriptase-polymerase chain reaction was applied to analyze the levels of expression of alpha, beta, and gamma -tubulin isotypes. Whereas alpha-tubulin isotype and gamma-tubulin tra nscripts were found to be expressed at constant levels throughout the cell cycle, some of the beta-tubulin isotype transcripts were found to be more highly expressed in dividing then in resting cells. Both of t he class IV beta-tubulin isotype transcripts (human 5 beta and beta 2, Class IVa and IVb, respectively) were expressed in dividing K562 and MES-SA cells at twice the levels found in resting cells. Increased exp ression of the class IV isotype proteins in dividing cells was confirm ed by immunoblotting, both in K562 and in MES-SA cells. A larger fract ion of total cell tubulin was found to be polymerized in dividing cell s (36-40%) than in resting cells (27-30%). The degree of polymerizatio n of class IV tubulin in dividing and resting cells was similar to tha t of total tubulin. These results show that total tubulin is expressed as constant levels throughout the cell cycle but that the degree of p olymerization is increased as cells are committed to division. The rel ative overexpression of the two class IV beta-tubulin isotypes in divi ding cells suggests functional specificity for these isotypes and a re gulatory role of these isotypes on the microtubule network during mito sis. (C) 1996 Wiley-Liss, Inc.