CHARACTERIZATION AND QUANTIFICATION OF FULL-LENGTH AND TRUNCATED NA,K-ATPASE ALPHA(1) AND BETA(1) RNA TRANSCRIPTS EXPRESSED IN HUMAN RETINAL-PIGMENT EPITHELIUM

We have characterized cDNA clones encoding the alpha(1) and beta(1) su bunits of Na,K-ATPase produced in the human retinal pigment epithelium (hRPE). In addition to isolating clones corresponding to known sequen ces of Na,K-ATPase subunits, we report hitherto unknown forms of Na,K- ATPase with unique deduced amino acid (aa) sequences in their C-termin i. Truncated cDNA sequences were found for both the beta(1) and alpha( 1) subunits. While the beta(1) sequence is truncated by two aa residue s at the C terminus, in the alpha(1) sequence 342 aa have been replace d by a unique sequence containing only 44 aa. Interestingly, this new C-terminal polypeptide shows sequence similarities to the Ca2+-ATPase and contains consensus sequence elements for phosphorylation and cell adhesion, suggesting expression of Na,K-ATPase subunits with unique fu nctions. Using reverse transcription-polymerase chain reaction, RNA se quences for alpha(1), beta(1) and their corresponding truncated isofor ms were quantified. 4.0 x 10(5) alpha(1) and 2.3 x 10(5) beta(1) molec ules were found per ng of mRNA from hRPE. Much lower levels were detec ted for truncated alpha(1) and beta(1) (3.6 x 10(3) and 2.7 x 10(3) mo lecules/ng, respectively). These data corroborate the expression of tr uncated transcripts coding for unique aa sequences in hRPE, and sugges t that factors other than alpha(1) and beta(1) mRNA levels regulate th e equimolar accumulation of alpha and beta subunits in the plasma memb rane.