The sarcoplasmic Ca²⁺-ATPase: design of a perfect chemi-osmotic pump

Abstract

The sarcoplasmic (SERCA 1a) Ca2+-ATPase is a membrane protein abundantly present in skeletal muscles where it functions as an indispensable component of the excitation-contraction coupling, being at the expense of ATP hydrolysis involved in Ca2+/H+ exchange with a high thermodynamic efficiency across the sarcoplasmic reticulum membrane. The transporter serves as a prototype of a whole family of cation transporters, the P-type ATPases, which in addition to Ca2+ transporting proteins count Na+, K+-ATPase and H+, K+-, proton- and heavy metal transporting ATPases as prominent members. The ability in recent years to produce and analyze at atomic (2·3-3 Å) resolution 3D-crystals of Ca2+-transport intermediates of SERCA 1a has meant a breakthrough in our understanding of the structural aspects of the transport mechanism. We describe here the detailed construction of the ATPase in terms of one membraneous and three cytosolic domains held together by a central core that mediates coupling between Ca2+-transport and ATP hydrolysis. During turnover, the pump is present in two different conformational states, E1 and E2, with a preference for the binding of Ca2+ and H+, respectively. We discuss how phosphorylated and non-phosphorylated forms of these conformational states with cytosolic, occluded or luminally exposed cation-binding sites are able to convert the chemical energy derived from ATP hydrolysis into an electrochemical gradient of Ca2+ across the sarcoplasmic reticulum membrane. In conjunction with these basic reactions which serve as a structural framework for the transport function of other P-type ATPases as well, we also review the role of the lipid phase and the regulatory and thermodynamic aspects of the transport mechanism.

Critical roles of isoleucine-364 and adjacent residues in a hydrophobic gate control of phospholipid transport by the mammalian P4-ATPase ATP8A2

“…On the basis of amino acid sequence alignment, the P4-ATPases are predicted to structurally resemble the classic P-type ATPase cation pumps Na + ,K + -ATPase and Ca 2+ -ATPase, possessing a transmembrane domain with 10 helices (M1-M10) and three cytoplasmic domains, P (phosphorylation), N (nucleotide binding), and A (actuator) (Fig. 1A), known from crystal structures to undergo large movements during the catalytic cycle (8,9). Recently, we showed that the P4-ATPase ATP8A2, like the cation pumps, forms an aspartyl-phosphorylated intermediate and undergoes a catalytic cycle involving the conformations E 1 , E 1 P, E 2 P, and E 2 similar to the Post-Albers scheme originally proposed for the Na + ,K + -ATPase ( Fig.…”

“…In the welldocumented catalytic cycles of Na + ,K + -ATPase and Ca 2+ -ATPase, the glutamate of M4 binds Na + /K + and Ca 2+ /H + and is alternately exposed to the two sides of the membrane during the cycle (9,17,18). Mutations of nearby located residues in M4 have been shown to block the E 1 P→E 2 P or E 2 P→E 2 transition (19)(20)(21).…”

Shifting hydrogen bonds may produce flexible transmembrane helices

“…If alternative hydrogen bonding patterns provide low-energy pathways for helix bending that are accessible by thermal motions, we might also expect to see hydrogen bond shifts during the conformational changes that occur in catalytic cycles. We therefore examined the backbone hydrogen bonding patterns that occur in the sarcoplasmic Ca 2þ -ATPase, for which there are many structures representing distinct stages in the transport cycle (34). As shown in Fig.…”

Membrane Anchoring and Ion-Entry Dynamics in P-type ATPase Copper Transport

“…The sulfur-sulfur distance between Met148 and the likely internal receiver, Cys382, is 9.3 Å in the Cu þ -free E2.P i state crystal structure (9), and although both residues showed significant flexibility in the E2.P i simulations, large-scale conformational changes are likely required to enable ion transfer, as observed in other P-type ATPase proteins (70). Therefore, although so far we had relied on the accuracy of the crystal structure, to characterize a putative ion-entry mechanism we had to accept the possible uncertainties of modeling an inward-facing LpCopA E1 state.…”