THE OVARIAN TUMOR PROTEIN ISOFORMS OF DROSOPHILA-MELANOGASTER EXHIBITDIFFERENCES IN FUNCTION, EXPRESSION, AND LOCALIZATION

The Drosophila melanogaster ovarian tumor (otu) gene, required for nor mal proliferation and differentiation of the female germ-line, encodes two cytoplasmic protein isoforms, 98 and 104 kDa. Mutants with defect s in this gene are typically grouped into one of three phenotypic clas ses: quiescent (germ cells do not proliferate), oncogenic or tumorous (germ-line cells proliferate uncontrollably), and differentiated (germ -line cells initiate but do not complete differentiation). Analysis of transformants expressing only one of the otu isoforms showed that the 104-kDa isoform (otu-104) can rescue all classes of otu mutants, wher eas only differentiated mutants are rescued to a significant extent by the 98-kDa isoform (otu-98). Western analysis of protein extracts pre pared from ovaries of various developmental stages indicated that otu- 104 predominates in predifferentiated stages, while otu-98 is prevalen t in differentiated egg chambers. Immunolocalization experiments demon strated that otu protein is present in the cytoplasm of oogonial stem cells that populate third instar larvae and in all germ-line-derived c ells until late in oogenesis. In stage 10 egg chambers, otu protein sh ifts to the subcortical region of nurse cells. This type of analysis a lso showed that upon formation of a 16-cell syncytium otu-104, but not otu-98, preferentially accumulates in the developing oocyte cytoplasm . The otu(11) mutant protein does not show this pattern of enhanced ac cumulation, nor does it occur in ovaries of egalitarian and Bicaudal-D mutants, which are defective in oocyte determination. Thus, these stu dies indicate that the 104-kDa isoform is required for normal prolifer ation of female germline cells and perhaps for oocyte differentiation. The 98-kDa isoform appears to be dispensable but can provide an otu f unction needed for the completion of oocyte maturation. (C) 1995 Acade mic Press,Inc.