Calcium-binding properties of two high affinity calcium-binding proteins from squid optic lobe.
We have studied the calcium-binding properties of two high affinity calcium-binding proteins from squid optic lobes: one, squid calmodulin (SCaM), similar to bovine brain calmodulin (BCaM), the other, squid calcium-binding protein (SCaBP), distinct (Head, J.F., Spielberg, S., and Kaminer, B. (1983) Biochem J. 209, 797-802). Equilibrium dialysis measurements on the squid proteins (and BCaM) were made at 100 mM KCl in the presence and absence of 3 mM Mg2+, and at 400 mM KCl in the presence of 3 mM Mg2+, which more closely resembles the conditions in the squid. SCaM, SCaBP, and BCaM each bind a maximum of 4 Ca2+ ions/molecule of protein under the ionic conditions tested. SCaBP has a higher affinity than SCaM or BCaM for Ca2+ at 100 mM KCl in the absence of Mg2+. However, in the presence of Mg2+, half-maximal binding to SCaBP occurs at a similar pCa value to that observed with calmodulin. Increasing the KCl concentration reduces the affinity of all three proteins for Ca2+. UV absorption measurements showed that the binding of 4 Ca2+ ions/molecule is necessary to complete spectral changes in SCaBP, compared to two for the calmodulins. While Ca2+ causes perturbations in aromatic chromophores in SCaM and SCaBP, Mg2+ causes a significant perturbation only in SCaBP. These Mg2+-induced changes differ qualitatively from those induced by Ca2+.
Sheldon
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Department of Physiology, Boston University School of Medicine, Massachusetts 02118.
Head
J F
JF
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002135:Calcium-Binding Proteins / Q000302:isolation & purification / Q000378:metabolism*; D002147:Calmodulin / Q000302:isolation & purification / Q000378:metabolism*; D049832:Decapodiformes; D007700:Kinetics; D008274:Magnesium / Q000378:metabolism / Q000494:pharmacology; D009899:Optic Lobe, Nonmammalian / Q000378:metabolism*; D013056:Spectrophotometry, Ultraviolet
D002135:Calcium-Binding Proteins; D002147:Calmodulin; D008274:Magnesium; D002118:Calcium