Decreases in calmodulin binding proteins and calmodulin dependent protein phosphorylation in the medial preoptic area at the onset of maternal behavior in the rat

Isolation and characterization of Dictyostelium thymidine kinase 1 as a calmodulin-binding protein

Probing of a cDNA expression library from multicellular development of Dictyostelium discoideum using a recombinant radiolabelled calmodulin probe (35S-VU1-CaM) led to the isolation of a cDNA encoding a putative CaM-binding protein (CaMBP). The cDNA contained an open reading frame of 951 bp encoding a 227aa polypeptide (25.5 kDa). Sequence comparisons led to highly significant matches with cytosolic thymidine kinases (TK1; EC 2.7.1.21) from a diverse number of species including humans (7e-56; 59% Identities; 75% Positives) indicating that the encoded protein is D. discoideum TK1 (DdTK1; ThyB). DdTK1 has not been previously characterized in this organism. In keeping with its sequence similarity with DdTK1, antibodies against humanTK1 recognize DdTK1, which is expressed during growth but decreases in amount after starvation. A CaM-binding domain (CaMBD; 20GKTTELIRRIKRFNFANKKC30) was identified and wild type DdTK1 plus two constructs (DdTK deltaC36, DdTK deltaC75) possessing the domain were shown to bind CaM in vitro but only in the presence of calcium while a construct (DdTK deltaN72) lacking the region failed to bind to CaM. Thus, DdTK1 is a Ca2+-dependent CaMBP. Sequence alignments against TK1 from vertebrates to viruses show that CaM-binding region is highly conserved. The identified CaMBD overlaps the ATP-binding (P-loop) domain suggesting CaM might affect the activity of this kinase. Recombinant DdTK is enzymatically active and showed stimulation by CaM (113+/-0.5%) an in vitro enhancement that was prevented by co-addition of the CaM antagonists W7 (91.2+/-0.8%) and W13 (96.6+/-0.6%). The discovery that TK1 from D. discoideum, and possibly other species including humans and a large number of human viruses, is a Ca2+-dependent CaMBP opens up new avenues for research on this medically relevant protein.

Calmodulin binds to and inhibits the activity of phosphoglycerate kinase

Phosphoglycerate kinase (PGK) functions as a cytoplasmic ATP-generating glycolytic enzyme, a nuclear mediator in DNA replication and repair, a stimulator of Sendai virus transcription and an extracellular disulfide reductase in angiogenesis. Probing of a developmental expression library from Dictyostelium discoideum with radiolabelled calmodulin led to the isolation of a cDNA encoding a putative calmodulin-binding protein (DdPGK) with 68% sequence similarity to human PGK. Dictyostelium, rabbit and yeast PGKs bound to calmodulin-agarose in a calcium-dependent manner while DdPGK constructs lacking the calmodulin-binding domain (209KPFLAILGGAKVSDKIKLIE228) failed to bind. The calmodulin-binding domain shows 80% identity between diverse organisms and is situated beside the hinge and within the ATP binding domain adjacent to nine mutations associated with PGK deficiency. Calmodulin addition inhibits yeast PGK activity in vitro while the calmodulin antagonist W-7 abrogates this inhibition. Together, these data suggest that PGK activity may be negatively regulated by calcium and calmodulin signalling in eukaryotic cells.

Calmodulin-binding domains in Alzheimer's disease proteins: extending the calcium hypothesis

The calcium hypothesis of Alzheimer's disease (AD) invokes the disruption of calcium signaling as the underlying cause of neuronal dysfunction and ultimately apoptosis. As a primary calcium signal transducer, calmodulin (CaM) responds to cytosolic calcium fluxes by binding to and regulating the activity of target CaM-binding proteins (CaMBPs). Ca(2+)-dependent CaMBPs primarily contain domains (CaMBDs) that can be classified into motifs based upon variations on the basic amphiphilic alpha-helix domain involving conserved hydrophobic residues at positions 1-10, 1-14 or 1-16. In contrast, an IQ or IQ-like domain often mediates Ca(2+)-independent CaM-binding. Based on these attributes, a search for CaMBDs reveals that many of the proteins intimately linked to AD may be calmodulin-binding proteins, opening new avenues for research on this devastating disease.

CaMBOT: profiling and characterizing calmodulin-binding proteins

Calmodulin (CaM) is an essential calcium-binding protein that binds to and activates a diverse population of downstream targets (calmodulin-binding proteins; CaMBPs) that carry out its critical signalling functions. In spite of the central importance of CaM in Ca(2+)-mediated signal transduction pathways in all eukaryotes, many CaMBPs remain to be identified and characterized. SDS-PAGE followed by gel overlay with recombinant, metabolically radiolabelled CaM (Calmodulin-binding Overlay Technique, CaMBOT) is a valuable method for following behavioural, developmental, forensic and physiological changes in total CaMBP populations and to identify candidate CaMBPs for further study. CaMBOT has also been adapted to isolate cDNAs encoding novel CaMBPs in various organisms. Recently, the method was used to examine the CaMBP complement encoded by the Arabidopsis genome and to identify a new family of transcription activators. To add to its diversity, CaMBOT may be useful for finding target proteins for work on phytoremediation and for the screening of pharmaceuticals and toxic agents that, directly or indirectly, affect CaM and its target proteins. This review discusses all of these topics and the role of CaMBOT in characterizing a functional unit of the proteome-proteins regulated by calmodulin.