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J. Biochem, 1993, Vol. 114, No. 2 203-209
© 1993 Japanese Biochemical Society

other

Uncoupling of ATP Splitting from Ca2+-Transport in Ca2+-Transporting ATPase of the Sarcoplasmic Reticulum as a Result of Modification by N-(3-Pyrene)Maleimide: Activation of a Channel with a Specificity for Alkaline Earth Metal Ions1

Taizo Suzuki and Masao Rawakita

Department of Physiological Chemistry, The Tokyo Metropolitan Institute of Medical Science Bunkyo-ku, Tokyo 113

The sarcoplasmic reticulum (SR) membranes of rabbit skeletal muscle were allowed to react with N-(3-pyrene)maleimide (PMI) at pH 7 at 30°C. The Ca2+-transporting activity of the SR membranes was reduced to 20% when PMI was bound to the extent of 1 mol/mol of Ca2+-transporting ATPase. The ATPase and the E-P forming activities were not affected by the binding of PMI up to 2 mol/mol ATPase, indicating that PMI somehow uncoupled Ca2+-transport from ATP splitting. Permeability of the SR membranes to Ca2+ ions was increased in parallel with the loss of the Ca2+-transporting activity. Of several components of the SR membranes which are reactive with PMI, the ATPase protein was the only one whose modification by PMI was directly related to the loss of the Ca2+-transporting activity. Similar results were obtained with the light SR membrane fraction, which lacks the ryanodine receptor, a well-recognized Ca2+ channel. These results indicated that a Ca2+ channel that would have been latent or properly regulated in native ATPase somehow escaped from the normal control mechanism as a result of modification of its SH groups by PMI and went into runaway operation. The activated channel was specific for alkaline earth metal ions, so permeability to other solutes including Co2+, Ni2+, and sucrose remained unchanged after treatment with PMI. Permeability of the PMI-activated ion channel was regulated by Ca2+ in the external medium at concentrations which would saturate the Ca2+-transporting sites, suggesting a functional coupling between the selective gate for the Ca2+-transport and this channel, which may be the passageway of Ca2+ during the active transport.

1 This study was supported in part by a Grant-in-Aid for Scientific Research on Priority Area of Molecular Structure and Regulation of Cellular Energy Coupling Systems from the Ministry of Education, Science and Culture of Japan.


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