Mutations in a β-Tubulin Disrupt Spindle Orientation and Microtubule Dynamics in the EarlyCaenorhabditis elegansEmbryo

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Abstract
The early Caenorhabditis elegans embryo contains abundant transcripts for two α- and two β-tubulins, raising the question of whether each isoform performs specialized functions or simply contributes to total tubulin levels. Our identification of two recessive, complementing alleles of a β-tubulin that disrupt nuclear-centrosome centration and rotation in the early embryo originally suggested that this tubulin, tbb-2, has specialized functions. However, embryos from tbb-2 deletion worms do not have defects in nuclear-centrosome centration and rotation suggesting that the complementing alleles are not null mutations. Both complementing alleles have distinct effects on microtubule dynamics and show allele-specific interactions with the two embryonically expressed α-tubulins: One of the alleles causes microtubules to be cold stable and resistant to the microtubule-depolymerizing drug benomyl, whereas the other causes cell cycle-specific defects in microtubule polymerization. Gene-specific RNA interference targeting all four embryonically expressed tubulin genes singly and in all double combinations showed that the tubulin isoforms in the early embryo are largely functionally redundant with the exception of tbb-2. tbb-2 is required for centrosome stabilization during anaphase of the first cell division, suggesting that tbb-2 may be specialized for interactions with the cell cortex.