Five Members of a Novel Ca2+-binding Protein (CABP) Subfamily with Similarity to Calmodulin

Abstract

Five members of a novel Ca2؉ -binding protein subfamily (CaBP), with 46 -58% sequence similarity to calmodulin (CaM), were identified in the vertebrate retina. Important differences between these Ca 2؉ -binding proteins and CaM include alterations within their second EF-hand loop that render these motifs inactive in Ca 2؉ coordination and the fact that their central ␣-helixes are extended by one ␣-helical turn. CaBP1 and CaBP2 contain a consensus sequence for N-terminal myristoylation, similar to members of the recoverin subfamily and are fatty acid acylated in vitro. The patterns of expression differ for each of the various members. Expression of CaBP5, for example, is restricted to retinal rod and cone bipolar cells. In contrast, CaBP1 has a more widespread pattern of expression. In the brain, CaBP1 is found in the cerebral cortex and hippocampus, and in the retina this protein is found in cone bipolar and amacrine cells. CaBP1 and CaBP2 are expressed as multiple, alternatively spliced variants, and in heterologous expression systems these forms show different patterns of subcellular localization. In reconstitution assays, CaBPs are able to substitute functionally for CaM. These data suggest that these novel CaBPs are an important component of Ca 2؉ -mediated cellular signal transduction in the central nervous system where they may augment or substitute for CaM.Among organisms as diverse as yeast and human, changes in the intracellular Ca 2ϩ ion concentration initiate an array of signaling pathways. Ca 2ϩ ions function as a diffusible signal that exerts its effect directly or through Ca 2ϩ -binding proteins on plasma membrane and intracellular channels, intracellular proteins involved in membrane trafficking, and a broad range of enzymes, including kinases, phosphatases, and adenylyl cyclases. Ca 2ϩ -binding proteins sense changes in [Ca 2ϩ ] through either 130-amino acid (aa) 1 structural elements called C2 domains, 29-aa EF-hand motifs, or through acidic regions of proteins or protein-lipid interfaces. In a growing number of eukaryotic signaling proteins, C2 and EF-hand motifs are present as either a single copy or clustered in multiple copies (1).The largest group of Ca 2ϩ -binding proteins belongs to the calmodulin (CaM) superfamily. They are structurally related and comprise four EF-hand motifs, some of which (one or two) may be nonfunctional in Ca 2ϩ coordination (2). Neuronal Ca 2ϩ -binding proteins (NCBP) are a subset of the EF-hand-containing proteins, whose function is largely unknown. The sequence similarity among members of the NCBP family varies from ϳ25% between CaM and visinin to ϳ60% between GCAP1 and GCAP3 (3). NCBPs are acidic and similar in length. CaM and CaM-like proteins are the shortest (149 -150 aa; molecular mass, 16,837 Da); other members of this family are ϳ200 aa long (molecular mass, ϳ23,000 Da) (2).NCBPs also display a variety of interesting structural features. Multifunctional CaM contains a pair of N-terminal (EFhand 1 and EF-hand 2) and C-terminal EF-hand (EF-hand 3 and EF-ha...

Essential role of Ca2+-binding protein 4, a Cav1.4 channel regulator, in photoreceptor synaptic function

“…14). The full-length cDNA of human CaBP4 contains an open reading frame of 825 bp, predicting a protein of 275 amino acids and a calculated molecular mass of 30,432 Da.…”

The Diversity of Calcium Sensor Proteins in the Regulation of Neuronal Function

“…In CaBPs 1 -5, the second EF-hand motif is inactive with the exception of CaBP3, which also has an inactive EF-1 motif. CaBP3 is believed to represent a pseudogene as only the mRNA has been detected in cells and as yet no protein product has been found (Haeseleer et al 2000). Two proteins were named CaBP7 and CaBP8 , but bioinformatic analysis is more consistent with them being a conserved and distinct subfamily of CaBPs (McCue et al 2010).…”

“…The CaBPs are a relatively recently discovered family of EF-hand containing Ca 2þ -binding proteins, which are only found in vertebrates (Haeseleer et al 2000). They represent another example of a diverse family of Ca 2þ -sensors capable of regulating discrete processes in the nervous systems of higher organisms.…”

“…The CaBPs also differ from calmodulin in that their central a helical linker domain connecting the carboxy-and amino-terminal EF-hand pairs is extended by four amino acid residues. This has been suggested to allow these proteins to interact with unique targets (Haeseleer et al 2000).…”

“…CaBP 1 and 2 are amino-terminally myristoylated, which permits localization to the plasma membrane and Golgi apparatus (Haeseleer et al 2000;Haynes et al 2004). The precise amino-terminal sequence to which the myristoyl group is attached is also important in the targeting of these two proteins, as exemplified by the long and short splice isoforms generated from their genes, which show subtle differences in their localization.…”