Glu415 in the α-tubulins plays a key role in stabilizing the microtubule–ADP-kinesin complexes

Abstract

Kavar21g, a dominant female-sterile mutation of Drosophila, identifies the αTubulin67C gene that encodes α4-tubulin, the maternally provided α-tubulin isoform. Although α4-tubulin is synthesized during oogenesis, its function is required only in the early cleavage embryos. However, once present in the developing oocyte, much of the α4-tubulin and the Kavar21g-encoded E426K-α4-tubulin molecules become incorporated into the microtubules. We analyzed ooplasmic streaming and lipid-droplet transport, with confocal reflection microscopy, in the developing egg primordia in the presence and absence of α4-tubulin and E426K-α4-tubulin and learnt that the E426K-α4-tubulin molecules eliminate ooplasmic streaming and alter lipid-droplet transport. Apparently, Glu426 is involved in stabilization of the microtubule-kinesin complexes when the kinesins are in the most labile, ADP-bound state. Replacement of Glu426 by Lys results in frequent detachments of the kinesins from the microtubules leading to reduced transport efficiency and death of the embryos derived from the Kavar21g-carrying females. Glu426 is a component of the twelfth α-helix, which is the landing and binding platform for the mechanoenzymes. Since the twelfth α-helix is highly conserved in the α-tubulin family, Glu415, which corresponds to Glu426 in the constitutively expressed α-tubulins, seems be a key component of microtubule-kinesin interaction and thus the microtubule-based transport.