A NERVOUS SYSTEM-SPECIFIC ISOTYPE OF THE BETA-SUBUNIT OF NA+,K+-ATPASE EXPRESSED DURING EARLY DEVELOPMENT OF XENOPUS-LAEVIS

We have previously described the isolation of several genes expressed exclusively in the nervous system of adult Xenopus laevis and activated in the embryo shortly after neural induction. The sequence of one of these cDNAs, 24-15, identifies the corresponding protein as an isotype of the β subunit of Na+,K+-ATPase [ATP phosphohydrolase (Na+/ K+-transporting); EC 3.6.1.37]. This form is distinct from the previously described β1 subunit of Xenopus, and the protein sequence comparison suggests that it is not the frog homolog of the mammalian β2 subunit; therefore, we refer to the 24-15 protein as the β3 subunit of Na+,K+-ATPase of Xenopus. Antisera directed against β3-subunit fusion protein detected a protein in adult brain extracts with the size and properties expected for a Na+,K+-ATPase β subunit. In Xenopus the β1 and β3 subunits are expressed as maternal mRNAs at similar levels; during embryogenesis rapid accumulation of β3 mRNA begins at stage 14 (early neurula), and the rapid accumulation of β1 mRNA begins at stage 23/24. In situ hybridization of antisense RNA probes to tadpole brain sections indicates that β3 subunit is expressed throughout the developing brain. We suggest that β3 is a major Na+,K+-ATPase β subunit present during early nervous system development in the frog.