Multiple and distinct effects of mutations of Tyr122, Glu123, Arg324, and Arg334 involved in interactions between the top part of second and fourth transmembrane helices in sarcoplasmic reticulum Ca2+-ATPase: changes in cytoplasmic domain organization during isometric transition of phosphoenzyme intermediate and subsequent Ca2+ release

We explored, by mutational substitutions and kinetic analysis, possible roles of the four residues involved in the hydrogen-bonding or ionic interactions found in the Ca2+-bound structure of sarcoplasmic reticulum Ca2+-ATPase, Tyr(122)-Arg(324), and Glu(123)-Arg(334) at the top part of second transmembrane helix (M2) connected to the A domain and fourth transmembrane helix (M4) in the P domain. The observed substitution effects indicated that Glu(123), Arg(334), and Tyr(122) contributed to the rapid transition between the Ca2+-unbound and bound states of the unphosphorylated enzyme. Results further showed the more profound inhibitory effects of the substitutions in the M4/P domain (Arg(324) and Arg(334)) upon the isomeric transition of phosphorylated intermediate (EP) (loss of ADP sensitivity) and those in M2/A domain (Tyr(122) and Glu(123)) upon the subsequent processing and hydrolysis of EP. The observed distinct effects suggest that the interactions seen in the Ca2+-bound structure are not functionally important but indicate that Arg(334) with its positive charge and Tyr(122) with its aromatic ring are critically important for the above distinct steps. On the basis of the available structural information, the results strongly suggest that Arg(334) moves downward and forms new interactions with M2 (likely Asn(111)); it thus contributes to the inclination of the M4/P domain toward the M2/A domain, which is crucial for the appropriate gathering between the P domain and the largely rotated A domain to cause the loss of ADP sensitivity. On the other hand, Tyr(122) most likely functions in the subsequent Ca2+-releasing step to produce hydrophobic interactions at the A-P domain interface formed upon their gathering and thus to produce the Ca2+-released form of EP. During the Ca2+-transport cycle, the four residues seem to change interaction partners and thus contribute to the coordinated movements of the cytoplasmic and transmembrane domains.