Inactivation of EF-hands makes GCAP-2 (p24) a constitutive activator of photoreceptor guanylyl cyclase by preventing a Ca2+-induced "activator-to-inhibitor" transition.
Guanylyl cyclase activator proteins GCAP-1 and GCAP-2 (Dizhoor et al. , 1995, Gorczyca et al., 1995) are members of a recently identified subclass of EF-hand type Ca2+-binding proteins that respond to Ca2+ differently than any other known members of the EF-hand superfamily. GCAPs acquire an activating conformation only in their Ca2+-free form. Free Ca2+ concentrations corresponding to levels in dark-adapted vertebrate photoreceptors inhibit the ability of GCAPs to activate photoreceptor guanylyl cyclases (RetGCs). We studied the effects of mutations that block binding of Ca2+ to the EF-hands of GCAP-2. Unlike other EF-hand proteins, which fail to activate their target when their EF-hands are inactivated by mutations, GCAP-2 with any single EF-hand inactivated remains active and is 3-6 times less sensitive to the inhibitory effect of Ca2+. Inactivation of any two or all three EF-hands produces active forms of GCAP-2 that are insensitive to inhibition by physiological intracellular concentrations of Ca2+. Unexpectedly we also found that activation of RetGCs by a Ca2+-insensitive mutant is inhibited by Ca2+-loaded wild type GCAP-2. We propose the following. 1) GCAP-2 can exist in two extreme functional forms: an apo form that activates RetGCs and a Ca2+-loaded form that blocks activation of RetGCs. 2) All three EF-hands of GCAP-2 contribute to the inhibitory effect of Ca2+. 3) Inactivation of two or three EF-hands is sufficient to shift the "activator-inhibitor" transition outside the physiological range of intracellular free Ca2+.
Dizhoor
A M
AM
Department of Biochemistry and Howard Hughes Medical Institute, University of Washington School of Medicine, Seattle, Washington 98195-7370, USA.
Hurley
J B
JB
D000595:Amino Acid Sequence; D000818:Animals; D001665:Binding Sites; D002118:Calcium / Q000378:metabolism*; D002135:Calcium-Binding Proteins / Q000235:genetics / Q000378:metabolism*; D002417:Cattle; D002460:Cell Line; D018076:DNA, Complementary; D004789:Enzyme Activation; D006162:Guanylate Cyclase / Q000378:metabolism*; D051604:Guanylate Cyclase-Activating Proteins; D008969:Molecular Sequence Data; D016296:Mutagenesis; D010786:Photoreceptor Cells / Q000201:enzymology*; D051381:Rats; D017386:Sequence Homology, Amino Acid
D002135:Calcium-Binding Proteins; D018076:DNA, Complementary; D051604:Guanylate Cyclase-Activating Proteins; D006162:Guanylate Cyclase; D002118:Calcium