Effects of various amino acid 256 mutations on sarcoplasmic/endoplasmic reticulum Ca2+ ATPase function and their role in the cellular adaptive response to thapsigargin.
Upon direct selection of mammalian cells for resistance to thapsigargin (TG), a potent inhibitor of the sarcoplasmic/endoplasmic reticulum Ca2+ transport ATPase (SERCA), the ATPase can acquire specific mutations at amino acid position 256 (aa256). In particular, Phe256 --> Leu and Phe256 --> Ser substitutions can occur upon TG selection, with each substitution resulting in a SERCA that is 4- to 5-fold resistant to TG inhibition (M. Yu et al., J. Biol. Chem. 273, 3542-3546, 1998). We have now identified a third substitution, i.e., Phe256 --> Val, that occurs when the Chinese hamster lung fibroblast cell line DC-3F is selected for TG resistance. Although the Phe256 --> Val substitution at codon 256 results in a SERCA whose enzymological properties in terms of Ca2+ transport and ATP hydrolysis are essentially similar to that of wild-type (wt) SERCA, the mutant enzyme is more than 40-fold resistant to TG inhibition. To analyze further the role of aa256 in TG-SERCA interactions, mutational analysis of this particular residue was also carried out. Of all the mutations introduced, only the Phe256 --> Glu substitution interferes with expression of the ATPase. The Phe256 --> Arg substitution does not interfere with SERCA expression, but the resulting enzyme is totally inactive. In terms of sensitivity of the various mutants to TG, maximal reduction in the ATPase's affinity for TG occurs with amino acid substitutions containing branched side chains, i.e. with the Phe256 --> Val, Phe256 --> Ile, and Phe256 --> Thr mutants. Since a corresponding Phe is conserved in the Na+, K+-ATPase which is not sensitive to TG, our findings suggest that this amino acid provides stabilization of the stalk segment with respect to the membrane interface, thereby optimizing specific interactions of TG with neighboring S3 residues (L. Zhong and G. Inesi, J. Biol. Chem. 273, 12994-12998, 1998). It is likely that a relatively high frequency of codon 256 mutations favor the aa256 mutants as a specific adaptive response to TG selection.