spe-39 was first identified as a mutant that usually arrests spermatogenesis at the spermatocyte stage. Although the cell division that forms spermatids (budding) is often attempted, it either fails or the buds are abnormally small. Mutations in
spe-39 are unlikely to interfere with meiosis because four condensed nuclei are often seen in the arrested spermatocytes. Electron microscopy suggests that the defects in cytokinesis is related to abnormalities in the morphogenesis of ER-Golgi-derived fibrous body-membranous organelle (FB-MO) complexes, specialized organelles that play an essential role in budding and packaging of cytoplasmic constituents required for sperm function. In wild type, each FB develops in close association with a MO. In contrast,
spe-39 mutants do not form MO's, so FB's exist as isolated structures in the cytoplasm. Additionally,
spe-39 spematocytes are full of tiny vesicles, whose formation is presumably related to the defects in MO morphogenesis. In order to understand the molecular basis of this interesting phenotype, we cloned
spe-39 by transgenic complementation. This gene encodes a 522-amino acid protein and, although it is involved in membrane morphogenesis, it lacks transmembrane region(s) according to hydrophobicity analysis. Northern analysis indicates that
spe-39 is not limited in its expression to the testis. Our RNAi results indicate that the gene is essential for somatic development and suggest that the null phenotype of
spe-39 is lethal.
spe-39 has orthologs in fly, mouse and human, but not in yeast, and the worm and human proteins are 24% identical. cDNA clones of the human gene are found in more than 20 libraries including brain, germ cell, muscle, embryo, skin and kidney. This suggests that
spe-39 and its orthologs play an essential role, perhaps in all metazoans. Presently, the subcellular localization of both the worm and human proteins is being established.