Mutations that encode partially functional β2 tubulin subunits have different effects on structurally different microtubule arrays

Abstract

The testis-specific β2 tubulin of Drosophila is required for assembly and function of at least three architecturally different microtubule arrays (Kemphues et al., 1982). Two recessive male-sterile mutations in the B2t locus that encode partially functional, stable, variant forms of β2 tubulin cause defects in only certain microtubule-based processes during spermatogenesis. These mutations could thus identify aspects of β tubulin primary structure critical for function only in specific microtubule arrays. In males carrying the B2t6 mutation, meiotic chromosome segregation and nuclear shaping are normal and flagellar axonemes are formed, but there is a subtle defect in axoneme structure; the outer doublet microtubules fill in with a central core normally seen only in the central pair and accessory microtubules. In homozygous B2t7 males, chromosome movement is usually normal during meiosis but cytokinesis often fails, cytoplasmic microtubules are assembled and nuclear shaping appears to be normal, but the flagellar axoneme lacks structural integrity. In contrast, the B2t8 allele affects a general property of tubulin, the ability to form normal side-to-side association of protofilaments (Fuller et al., 1987), and causes defects in meiosis, axoneme assembly and nuclear shaping. Certain combinations of these β2 tubulin mutations show interallelic complementation; in B2t6/B2t8 males functional sperm are produced and both variant subunits are incorporated into mature sperm, in the absence of wild-type β2 tubulin. Comparison of the phenotypes of the three partially functional β2 tubulin alleles reveals some aspects of tubulin primary structure more important for function in specific subsets of microtubule arrays, and other aspects required for the construction of microtubules in general.