THE PUTATIVE AMINO-TERMINAL SIGNAL PEPTIDE OF THE CLONED RAT-BRAIN NA-CA2+ EXCHANGER GENE (RBE-1) IS NOT MANDATORY FOR FUNCTIONAL EXPRESSION()

The rat brain Na+-Ca2+ exchanger (RBE) gene, as well as other isoforms of this protein family, can be organized into 12 transmembrane alpha helices, the first of which was proposed by Durkin et al. (14) to cons titute a cleavable signal peptide. We have prepared three amino-termin al mutants, in which 21, 26, and 31 amino acids beyond the initiating methionine were deleted. The deletions include the hydrophobic core of the putative signal peptide (N21), the entire putative signal peptide and parts of the putative signal peptidase cleavage site (N26), and t he entire putative signal peptide and putative signal peptidase cleava ge site (N31). Ah three mutant clones were transiently expressed in He La cells. The average Na+ gradient-dependent Ca2+ transport activity o f the mutant exchangers was 108% (N21), 37.2% (N26), and 60.06% (N31) of the wild-type clone. Mutation of the putative cleavage site by an e xchange of Ala-32 --> Asp, resulted in a decrease in Na+-Ca2+ exchange activity to 7.7%, relative to the wild-type exchanger. Functional rec onstitution of the proteins that were expressed in the transfected cel ls, resulted in transport activities of: 60.1% (N21), 26.75% (N26), 85 .36% (N31), and 31% (Ala-32 --> Asp) relative to the wild-type exchang er. Western blot analysis of the protein profile of RBE-I, N21, N26, N 31 and Ala-32 --> Asp-transfected HeLa cells was carried out by using an antipeptide antibody directed against a pentadecapeptide segment de rived from the large putative cytoplasmic loop of the cloned rat excha nger gene. In the total cell extract and in the plasma membrane-enrich ed fraction, in addition to a major protein band of about 125 kDa, whi ch corresponds to the molecular mass of the mature fully processed Na-Ca2+ exchanger, an additional protein of about 135 kDa is revealed in the profile of N21- and N26-transfected cells. This band is not detec ted in the protein profile of RBE-1, N31, or Ala-32 --> Asp. The amino -terminal truncated mutants of the cloned Na+-Ca2+ exchanger could be expressed and processed also in a reticulocyte lysate supplemented wit h dog microsomes. Our results suggest that the putative signal peptide of the cloned Na+-Ca2+ exchanger gene does not play a mandatory role in functional expression of the protein in HeLa cells.