Localized Products of futile cycle/ lrmp Promote Centrosome-Nucleus Attachment in the Zebrafish Zygote

Abstract

The centrosome has a well-established role as a microtubule organizer during mitosis and cytokinesis. In addition, it facilitates the union of parental haploid genomes following fertilization by nucleating a microtubule aster along which the female pronucleus migrates toward the male pronucleus. Stable associations between the sperm aster and the pronuclei are essential during this directed movement. Our studies reveal that the zebrafish gene futile cycle (fue) is required in the zygote for male pronucleus-centrosome attachment and female pronuclear migration. We show that fue encodes a novel, maternally-provided long form of lymphoid-restricted membrane protein (lrmp), a vertebrate-specific gene of unknown function. Both maternal lrmp messenger RNA (mRNA) and protein are highly localized in the zygote, in a largely overlapping pattern at nuclear membranes, centrosomes, and spindles. Truncated Lrmp-EGFP fusion proteins identified subcellular targeting signals in the C terminus of Lrmp; however, endogenous mRNA localization is likely important to ensure strict spatial expression of the protein. Localization of both Lrmp protein and lrmp RNA is defective in fue mutant embryos, indicating that correct targeting of lrmp gene products is dependent on Lrmp function. Lrmp is a conserved vertebrate gene whose maternally inherited products are essential for nucleus-centrosome attachment and pronuclear congression during fertilization. Precise subcellular localization of lrmp products also suggests a requirement for strict spatiotemporal regulation of their function in the early embryo.