A SINGLE GAMMA-TUBULIN GENE AND MESSENGER-RNA, BUT 2 GAMMA-TUBULIN POLYPEPTIDES DIFFERING BY THEIR BINDING TO THE SPINDLE POLE ORGANIZING CENTERS

Cells of eukaryotic organisms exhibit microtubules with various functi ons during the different developmental stages, The identification of m ultiple forms of alpha- and beta-tubulins had raised the question of t heir possible physiological roles, In the myxomycete Physarum polyceph alum a complex polymorphism for alpha- and beta-tubulins has been corr elated with a specific developmental expression pattern, Here, we have investigated the potential heterogeneity of gamma-tubulin in this org anism. A single gene, with 3 introns and 4 exons, and a single mRNA co ding for gamma-tubulin were detected, They coded for a polypeptide of 454 amino acids, with a predicted molecular mass of 50,674, which pres ented 64-76% identity with other gamma-tubulins, However, immunologica l studies identified two gamma-tubulin polypeptides, both present in t he two developmental stages of the organism, uninucleate amoebae and m ultinucleate plasmodia, The two gamma-tubulins, called gamma(s)- and g amma(f)-tubulin for slow and fast electrophoretic mobility, exhibited apparent molecular masses of 52,000 and 50,000, respectively, They wer e recognized by two antibodies (R70 and JH46) raised against two disti nct conserved sequences of gamma-tubulins, They were present both in t he preparations of amoebal centrosomes possessing two centrioles and i n the preparations of plasmodial nuclear metaphases devoid of structur ally distinct polar structures, These two gamma-tubulins exhibited dif ferent sedimentation properties as shown by ultracentrifugation and se dimentation in sucrose gradients, Moreover, gamma(s)-tubulin was tight ly bound to microtubule organizing centers (MTOCs) while gamma(f)-tubu lin was loosely associated with these structures, This first demonstra tion of the presence of two gamma-tubulins with distinct properties in the same MTOC suggests a more complex physiological role than previou sly assumed.