CaM-stimulated Ca2+-ATPase activity on the symbiosome membrane from broad bean root nodules and its sensitivity to chlorpromazine
2021
Krylova, Valeriya V. | Zartdinova, Rosariya | Izmailov, Stanislav F.
Relatively recently, we provided preliminary evidence for stimulating action of CaM on the activity of the symbiosome membrane (SM) Ca²⁺-ATPase from broad bean root nodules functioning, as to date found by us, as ATP-driven Ca²⁺/nH⁺ exchanger. In the present work, CaM effects on the enzyme studied in much more detail by following its catalytic and transport activities. It was found that the kinetics of both ITP hydrolysis by the SM vesicles and symbiosomes and two processes involved in the transport cycle of the Ca²⁺-ATPase, namely, Ca²⁺ uptake by symbiosomes and development within them of alkaline pH shift, is markedly accelerated in the presence of CaM and inhibited by its antagonist, chlorpromazine (CP). Based on the data obtained, it was concluded that under selected experimental conditions the observed modulation by CaM of the calcium pump activities is largely caused by the increase in Vₘₐₓ of the enzyme rather than its apparent Kₘ for Ca²⁺. These results are discussed in the light of the available literature data showing that the mechanism of CaM action on Ca²⁺-ATPases in eukaryotic cell membranes may be based not only on dissociation in the presence of Ca²⁺-CaM complex of the CaM-binding auto-inhibitory domain from their active center but also the use of some other mechanisms not involving any significant structural changes of CaM-binding target proteins.
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