Substrate-induced conformational fit and headpiece closure in the Ca2+ATPase (SERCA)

Protection of the Ca2+ATPase (SERCA) from proteinase K digestion has been observed following the addition of Ca2+, Mg2+, and nucleotide and interpreted as a substrate-dependent conformational change (1). The protected digestion site is located on the loop connecting the A domain and the M3 transmembrane helix. We studied by mutational analysis the protective effect of AMP-PCP, an ATP analog that is not utilized for enzyme phosphorylation. We found that the nucleotide protective effect is interfered with by single mutations of Arg-560 and Glu-439 in the N domain and Lys-352, Lys-684, Thr-353, Asp-703, and Asp-707 in the P domain. This is consistent with a transition from the open to the compact configuration of the ATPase headpiece and approximation of the N and P domains by interactions with the nucleotide adenosine and phosphate moieties, respectively. The A domain-M3 loop is consequently involved. Protection by nucleotide substrate increased following the mutations of Asp-351 (the residue undergoing phosphorylation by ATP) and neighboring Asn-706 to Ala, underlying the importance of side chain specificity in positioning the nucleotide terminal phosphate and limiting the stability of the substrate-enzyme complex. Protection is not observed when AMP-PCP is added in the absence of Ca2+ or following mutations (E771Q or N796A) that interfere with Ca2+ binding. Therefore, nucleotide binds to the Ca2+-activated enzyme in the open headpiece conformation and the consequent approximation of the N and P domains occurs while the transmembrane domain is still in the Ca2+-bound conformation. Mg2+ is not required for the protective effect of nucleotide, even though it is specifically required for the subsequent catalytic reactions.