Abstract
Immunofluorescence staining of Drosophila embryos with a monoclonal antibody specific for acetylated α tubulin has revealed that acetylated and nonacetylated a-tubulin isoforms have different patterns of distribution during early development. Acetylated α-tubulin was not detected in either interphase or mitotic spindle microtubules during the rapid early cleavage or syncytial blastoderm divisions. Acetylated α-tubulin was first observed as interphase lengthened at the end of syncytial blastoderm, and at cycle 14 was localized to a ring of structures clustered around the interphase nuclei. These structures probably represent a set of stable microtubules involved in nuclear elongation. Absence of detectable acetylated α-tubulin prior to cellular blastoderm seems to be due to rapid turnover of microtubule arrays rather than to lack of the enzyme required for modification, since acetylated α-tubulin appeared in early embryos when microtubules were stabilized by taxol treatment or anoxia. Because acetylated α-tubulin seems to be characteristic of stable microtubule arrays, the appearance of the antigen at cycle 14 represents a fundamental change in microtubule behaviour in the somatic cells of the embryo. Acetylated α-tubulin was not detected in pole cells during the blastoderm or early gastrula stages, indicating that acetylation of α-tubulin is not merely a consequence of cellularization.
After the onset of gastrulation, interphase microtubule arrays in most cell types contain acetylated α-tubulin. However, cells in mitosis lack antibody staining. The resulting unstained patches reveal the stereotyped spatial pattern of cell division during gastrulation. Although the cells that give rise to the amnioserosa have acetylated α-tubulin in their interphase arrays at early gastrulation, by germ band elongation these large, plastic cells completely lack staining with anti-acetylated α-tubulin. In contrast, differentiated cell types such as neurones, which have arrays of stable axonal microtubules, stain brightly with the specific antibody. Although acetylated and nonacetylated α-tubulin are present in roughly equal amounts by the late stages of embryogenesis, acetylated α-tubulin is partitioned into the pellet during centrifugation of extracts of embryos homogenized at 4°C.
