Isolation of microtubule motors from an insect ovarian system: characterization using a novel motility substratum

Abstract

The ovaries of hemipteran insects contain massive microtubule-based translocation channels known as nutritive tubes, linking nurse cells to the developing oocytes. Translocation, which is in a retrograde direction along the nutritive tube microtubules, has previously been reactivated in vitro. Here, ATPsensitive microtubule-associated proteins (MAPs) have been isolated from the insect ovaries, and beads coated with such proteins applied to salt-treated, detergent-extracted nutritive tube microtubules microdissected from the insect ovaries. These motility substrata are composed of many thousands of parallel microtubules, all with a common known polarity, so that not only are they easily observed, but the direction of any translocation along their length can be readily interpreted. ATP extracts of insect ovarian MAPs, containing both kinesin and dynein, were seen to promote bidirectional movements of beads. Movements in the two directions differed in both rate and form. On fractionation of the ATP extract, those fractions containing kinesin brought about bead movement in an anterograde direction. Fractions containing dynein failed to promote movement of beads, and no single fraction promoted movement of beads in a retrograde direction. Kinesin, while clearly present in the insect ovary, is absent from the nutritive tube translocation channels. The nutritive tubes, however, contain a polypeptide that co-electrophoreses with insect ovarian dynein, making dynein a possible candidate for the motor that drives the retrograde translocation along nutritive tubes.