POLYADENYLATION OF NA-K+-ATPASE BETA(1)-SUBUNIT DURING EARLY DEVELOPMENT OF XENOPUS-LAEVIS()

In fully grown Xenopus oocytes, the synthesis of beta-subunits is limi ting for the formation of functional Na+-K+-adenosinetriphosphatase ai p-complexes (Geering, K. FEBS Lett. 285: 189-193, 1991). In the presen t study, we show that during oocyte growth (from stage I to stage VI) alpha(1)-, but not beta(1)- or beta(3)-isoform, mRNAs accumulate. In a ddition, beta-mRNAs are apparently sequestered in an untranslated pool in fully grown oocytes (stage VI). From fertilization to morulation, the total pools of alpha(1)-, beta(1)-, or beta(3)-mRNAs vary little. Whereas polyadenylated [poly(A)(+)] alpha(1)- and beta(3)-isoform mRNA s did not change significantly, poly(A)(+) beta(1)-mRNA abundance incr eased three- to fourfold at morulation, accompanied by a parallel incr ease in beta(1)-protein synthesis. After midblastula transition (i.e., at early gastrula) and during neurulation, poly(A)(+) alpha(1)- and b eta(3)-mRNAs accumulated rapidly, whereas poly(A)(+) beta(1)-mRNA accu mulation was delayed by similar to 2 h, beginning only at early neurul a. Our results indicate that 1) the abundance of poly(A)(+) beta(1)-mR NA is rate limiting during embryonic development for the assembly of a lpha(1)/beta(1)-heterodimers, shown to be involved in the vectorial tr ansport of sodium in kidney cells, and 2) the polyadenylation of beta( 1)-mRNA is a rate-limiting factor during morulation for the synthesis and assembly of new sodium pumps at the time of blastocoel fluid forma tion. The 3'-untranslated region of beta(1)-mRNA (but not of alpha(1)- mRNA) expresses cytoplasmic polyadenylation elements (CPEs) with the c onsensus sequence AXXAUUUU(A/U)(A/U)(A/U). A role of CPE in the differ ential polyadenylation of alpha(1)- and beta(1)-mRNA is proposed.