Interaction of an Overexpressed gamma-Tubulin with Microtubules In Vivo and In Vitro

gamma-Tubulin is an ubiquitous MTOC (microtubule-organizing center) component essential for the regulation of microtubule functions. A 1.8 kb cDNA coding for gamma-tubulin was isolated from CHO cells. Analysis of nucleotide sequence predicts a protein of 451 amino acids, which is over 97% identical to human and Xenopus gamma-tubulin. When CHO cells were transiently transfected with the gamma-tubulin clone, epitope-tagged full-length, as well as truncated polypeptides (amino acids 1-398 and 1-340), resulted in the formation of cytoplasmic foci of various sizes. Although one of the foci was identified as the centrosome, the rest of the dots were not associated with any other centrosomal components tested so far. The pattern of microtubule organization was not affected by induction of such gamma-tubulin-containing dots in transfected cells. In addition, the cytoplasmic foci were unable to serve as the site for microtubule regrowth in nocodazole-treated cells upon removal of the drug, suggesting that gamma-tubulin-containing foci were not involved in the activity for microtubule formation and organization. Using the monomeric form of Chlamydomonas gamma-tubulin purified from insect Sf9 cells (), interaction between gamma-tubulin and microtubules was further investigated by immunoelectron microscopy. Microtubules incubated with gamma-tubulin monomers in vitro were associated with more gold particles conjugated with gamma-tubulin than in controls where no exogenous gamma-tubulin was added. However, binding of gamma-tubulin to microtubules was not extensive and was easily lost during sample preparation. Although gamma-tubulin was detected at the minus end of microtubules several times more frequently than the plus end, the majority of gold particles were seen along the microtubule length. These results contradict the previous reports (; ), which might be ascribed to the difference in the level of protein expression in transfected cells.

Function of a minus-end-directed kinesin-like motor protein in mammalian cells

CHO2 is a mammalian minus-end-directed kinesin-like motor protein present in interphase centrosomes/nuclei and mitotic spindle fibers/poles. Expression of HA- or GFP-tagged subfragments in transfected CHO cells revealed the presence of the nuclear localization site at the N-terminal tail. This domain becomes associated with spindle fibers during mitosis, indicating that the tail is capable of interaction with microtubules in vivo. While the central stalk diffusely distributes in the entire cytoplasm of cells, the motor domain co-localizes with microtubules throughout the cell cycle, which is eliminated by mutation of the ATP-binding consensus motif from GKT to AAA. Overexpression of the full-length CHO2 causes mitotic arrest and spindle abnormality. The effect of protein expression was first seen around the polar region where microtubule tended to be bundled together. A higher level of protein expression induces more elongated spindles which eventually become disorganized by loosing the structural integrity between microtubule bundles. Live cell observation demonstrated that GFP-labeled microtubule bundles underwent continuous changes in their relative position to one another through repeated attachment and detachment at one end; this results in the formation of irregular number of microtubule focal points in mitotic arrested cells. Thus the primary action of CHO2 appears to cross-link microtubules and move toward the minus-end direction to maintain association of the microtubule end at the pole. In contrast to the full-length of CHO2, overexpression of neither truncated nor mutant polypeptides resulted in significant effects on mitosis and mitotic spindles, suggesting that the function of CHO2 in mammalian cells may be redundant with other motor molecules during cell division.