MOVEMENT OF NUCLEI ALONG MICROTUBULES IN XENOPUS EGG EXTRACTS

Microtubules are implicated in the movement and positioning of nuclei in many cell types, Nuclei can be moved by forces acting on microtubul es nucleated at the spindle pole body, as in fungi [1], or microtubule s nucleated at the centrosome, as during migration of the male (sperm) pronucleus towards the centre of the zygote after fertilization [2-4] , The dramatic movements of the female pronucleus towards the male pro nucleus potentially involve another mechanism: movement along the micr otubules of the sperm aster towards their slower growing, attached or 'minus' ends [.3,5]. Here, we have reconstituted this last type of nuc lear movement in vitro. Synthetic nuclei assembled in cytoplasmic extr acts made from interphase Xenopus eggs move along microtubules towards their minus ends, We provide strong experimental evidence that cytopl asmic dynein is the motor for nuclear movement in this in vitro system , and discuss our results in terms of current knowledge of motility of the endoplasmic reticulum.