CC-115 receptor br Structural studies of GCAP Recoverin was
Structural studies of GCAP Recoverin was the first member of this subfamily of Ca2+-binding CC-115 receptor to be investigated by structural methods , , , , , . These proteins have four EF-hands arranged in pairs that give rise to amino- and carboxyl-terminal domains. The crystal structure of recoverin displays a 3-D arrangement that is different from that of CaM. The relative positions of the N- and C-terminal domains form a more compact structure than that found in CaM (Fig. 4). In CaM, the two domains are linked by a long helix generating a dumbbell-like molecule, whereas in recoverin a more compact conformation is adopted in which the bent central helices bring together the two domains. This compact conformation is shared with other Ca2+-binding proteins such as neurocalcin and frequenin (revised in ). As seen in recoverin, the structure of GCAP2 as determined in solution by NMR reveals that this protein contains four EF-hand motifs arranged in a compact tandem array. In comparing the Ca2+-bound structures of GCAP2 and recoverin , the root mean square deviation of the main chain atoms in the EF-hand regions is 2.2Å.
Perspectives The problem of how GCAPs convert GCs from low to high activity enzymes at the molecular level will likely be answered by mutagenesis and crystallography. Some GCAPs, perhaps GCAP2 more than others, may play additional roles other than GC stimulation. This may be particularly true for GCAP3 through GCAP8, which have recently been identified in teleost. As for the third issue, high resolution structural determinations will be accomplished for GCAPs as they have for recoverin, neurocalcin, frequenin, and GCAP2 (reviewed in , ). Structural studies of the complex of GC-GCAP  by co-crystallization will follow, and the regulation of photoreceptor GC by GCAPs has a chance to become a fully understood system at the molecular level in the near future.