CHANGES IN NUCLEAR-LOCALIZATION OF AN3, A RNA HELICASE, DURING OOGENESIS AND EMBRYOGENESIS IN XENOPUS-LAEVIS

The immunolocalization of An3 protein, an ATP-dependent RNA helicase a nd a member of the DEAD box family, was compared with the localization of fibrillarin, a protein essential for rRNA processing, and snRNPs, which are involved in mRNA splicing reactions, during oogenesis and em bryogenesis in Xenopus laevis. Although An3 protein was detected in th e cytoplasm of all stages of oocytes, in most stages An3 protein was a lso present in the nucleus. Prior to stage I An3 protein was uniformly dispersed throughout the entire germinal vesicle; from stages I to V it was in nucleoli. By stage VI nucleolar labeling with anti-An3 disap peared and the protein was no longer present within nuclei. An3 reacti vity was also present throughout the nuclei of follicle cells surround ing prestage I to stage VI oocytes. Both cytoplasmic and nuclear An3 s taining were present in cells of stages 8 to 35 embryos; however, nucl ear staining was punctate and uniformly distributed throughout the nuc leoplasm. Fibrillarin was diffusely distributed throughout the entire germinal vesicle prior to stage I, localized exclusively to nucleoli o f oocytes between stages I and VI and in nucleoli of stages 12 and 35 embryonic cells. Reactivity for snRNPs (anti-Sm) in germinal vesicles of prestage I oocytes was diffuse, and similar to the distribution of An3 and fibrillarin; in later stage oocytes anti-Sm staining was restr icted to a population of granules, much fewer in number and more heter ogeneous in size than nucleoli. Anti-Sm activity was apparent in nucle i of embryonic cells of stages 8 to 35 embryos. Although colocalizatio n of the Sm epitope and An3 was not observed in developing oocytes and in embryonic cells, Sm reactive material was frequently found in clos e association with An3-positive nucleoli (oocytes) and nuclear deposit s (embryonic cells). In stage IV and V oocytes treated with actinomyci n D (4 mu g/ml) to inhibit rRNA synthesis, nucleoli, which continued t o possess fibrillarin, lacked An3; staining of follicle cell nuclei fo r An3 was unchanged. Treatment with 200 mu g/ml actinomycin D to block mRNA synthesis, inhibited An3 but not fibrillarin staining in nuclei of prestage I oocytes and follicle cells. The changing patterns of An3 reactivity and the differential effects of actinomycin D on such loca lizations observed here are consistent with a role for An3 in the proc essing/production of RNA. (C) 1996 Wiley-Liss, Inc.