Roles of Leu249, Lys252, and Leu253 in Membrane Segment M3 of Sarcoplasmic Reticulum Ca2+-ATPase in Control of Ca2+ Migration and Long-Range Intramolecular Communication

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Abstract
Point mutants with alterations to Leu249, Lys252, Leu253, Asp254, and Glu255 in membrane segment M3, and Pro824, Lys825, and Glu826 in loop L6−7, of the sarcoplasmic reticulum Ca2+-ATPase were analyzed functionally by steady-state and transient kinetic methods. In mutants Leu249Ala, Lys252Glu, and Leu253Ala, the rate of Ca2+ dissociation from the cytoplasmically facing high-affinity Ca2+ sites was increased 4- to 7-fold relative to wild type, and in Leu249Ala and Lys252Glu the rate of Ca2+ binding was increased as well. Substitution of Lys252 with arginine, alanine, glutamine, or methionine affected Ca2+ interaction much less, indicating that the negative charge of the glutamate is particularly disturbing. These findings may be understood on the basis of the hypothesis that a water-accessible channel leading between membrane segments M1 and M3 in the thapsigargin-bound Ca2+-free structure [Toyoshima, C., and Nomura, H. (2002) Nature418, 605−611] is closely related to the migration pathway for Ca2+. The effects of alanine mutations to Leu249 and Leu253 on Ca2+ dissociation may arise from destabilization of the hydrophobic wall lining the pathway. In mutant Lys252Glu, unfavorable interaction between the glutamate and L6−7 may open the pathway. In addition, Leu253Ala, and to a lesser extent some of the other mutations, reduced the rate of the E1PCa2 to E2P transition of the phosphoenzyme, enhanced the rate of dephosphorylation of E2P, and reduced the apparent affinity for vanadate, suggesting interference with the conformational change of the phosphoenzyme and the function of the catalytic site in E2 and E2P.