Phosphorylation Domain

Transmembrane Domain

Themost important landmark of the transmembrane domain is the calciumbinding site, which in the E1 conformation cooperatively binds two calcium ions from the cytoplasm (Figure 3a). The associated residues correspond remarkably well with those previously identified by site-directed mutagenesis. Initially, these residues were identified by phosphorylation of mutant pumps (15), which require calcium,bindingwhen performed in the forward directionwithATP but notwhen performed in reverse using Pi. Later, the sequential nature of calcium binding was used to distinguish the two sites because binding by the first calcium ion was sufficient to prevent phosphorylation from Pi, whereas binding by both calcium ions was required for phosphorylation from ATP. Thus, individual calcium site mutants displaying Pi phosphorylation that was insensitive to calcium were assigned to the first site, whereas those with normal sensitivity were assigned to the second site (3). This analysis was later corroborated by direct measurement of calcium binding stoichiometries in a more efficient expression system (92). Specifically, the X-ray structure showed oxygen ligands for calcium provided by residues on M4, M5, M6, and M8. N768 and E771 on M5, T799 and D800 on M6, and E908 on M8 bound the first calcium ion. The contribution of adjacent residues along M6 was aided by flexibility in this helix due to the nonhelical hydrogen bonding of carbonyl oxygens of N799 and D800. The second ion binding site was quite different, with extensive contributions from main chain carbonyl oxygens along M4 as well as side chain oxygens from E309 on M4 and N796 and D800 on M6. A highly conserved sequence motif on M4 (PEGL311) lies at the heart of this second site and likely represents the key to cooperativity. In particular, binding of the first ion to M5/M6/M8 must somehow induce the favorable configuration of M4 to provide for the cooperative binding of the second ion. This implied structural flexibility of M4, as well as its direct link to the phosphorylation site, places the separticular structural elements at the center of the global conformational change that accompanies calcium binding to the enzyme.

Phosphorylation Domain

Phosphorylation occurs on an aspartate»30 residues beyond the C-terminal end of M4 in a highly conserved region that serves as a signature sequence for P-type ATPases: DKTGT355. Initially, this phosphorylation site was identified by chemical means; later, site-directed mutagenesis of the aspartate and its conserved neighbors was shown to interfere specifically with phosphorylation (2). The fold of the P domain had previously been deduced by analogy with bacterial dehalogenases and had been used to define a superfamily of hydrolases that also included small-molecule phosphatases (5). This deduction relied on an alignment of several short, highly conserved sequences that play key roles in the catalytic sites of all these enzymes: DKTGT355, KSK686, TGD627, and DGVND707 (Figure 1). The resulting structural prediction consisted of a Rossmann fold with an inserted ATP binding domain and was consistent with predictions of secondary structure and with effects of mutagenesis and chemical modification throughout both domains (90). A common catalytic mechanism was also implied by the fact that the nucle- ophilic aspartates (D351) of both dehalogenases and phosphatases form a covalent intermediate during the reaction cycle (16). These predictions were confirmed by the X-ray structure of Ca2C-ATPase, which revealed not only a Rossmann fold but also a common arrangement of catalytic site residues (Figure 4) relative to both dehalogenases (39) and phosphatases (100). Although the Rossmann fold represents the template for this phosphorylation domain, Ca2C-ATPase has several adaptations relevant to the energy coupling re- quired for calcium transport. The fold is characterized by a central, six-stranded parallel ¯-sheet flanked by three ®-helices on each side (Figures 1 and 4). As predicted from the sequence of Ca2C-ATPase, the ®-helices alternate with the¯-strands along the peptide chain. Typical of ®/¯ structures, the active site exists at the topological break point dividing the first three strands from the last three strands, and critical residues appear in the loops between each strand and the subsequent helix (10). The N domain interrupts the loop following the phosphorylated aspartate which contains the most highly conserved sequences of the family, namely the signature sequence DKTGT355 following the phosphorylation site and DPPR604 in the return from the N domain. The P domain is firmly connected to the transmembrane domain by cytoplasmic extensions ofM4 andM5,which constitute so-called stalk helices (S4 and S5). In the case of S5, its further cytoplasmic extension represents one of six flanking helices of the Rossmann fold. S4 representsan extra structural element, which is followed by a short, antiparallel ¯-strand and ®-helix leading to the beginning of the Rossmann fold. This preliminary ¯-strand extends the central ¯-sheet and probably serves to couple movements of M4 to those of the phosphorylation domain. The preliminary ®-helix, dubbed P1, runs underneath the Rossmann fold and interacts with lower parts of S5 as well as with the loop between M6 and M7, thus potentially coupling movements of the membrane components with those of the phosphorylation domain. Finally, there is aninsert consisting of a strand and two short helices (P4a and P4b) on the other end of the Rossmann fold. This insert is on the periphery of the structure and is highly variable among P type ATPases, being considerably larger in NaC/KC-ATPase and absent in CadA (90).

Дата добавления: 2015-10-29