Calcium/calmodulin inhibition of basic-helix-loop-helix transcription factor domains

Abstract

The ubiquitous Ca(2+)-binding protein calmodulin (CaM) is a key protein in Ca2+ homeostasis and activation of eukaryotic cells. CaM is the molecular link between free Ca2+ in the cell and the inhibition, or activation, of numerous enzymes. Many nuclear functions are under Ca2+/CaM control, and some transcriptional activators are known to be Ca2+ modulated indirectly through Ca2+/CaM-dependent protein kinases. But Ca2+/CaM has not yet been found to directly modulate any transcription factor or other DNA-binding protein. Transcription factors of the basic-helix-loop-helix (bHLH) group are important regulators in numerous systems. Here we report that binding of Ca(2+)-loaded CaM to the bHLH domains of several bHLH proteins directly inhibits their DNA binding. Other bHLH proteins are either less sensitive or resistant. Ca2+ ionophore selectively inhibits transcriptional activation by Ca2+/CaM-sensitive bHLH proteins in vivo, implying that Ca2+ can directly influence transcription through differential CaM inhibition of bHLH domains.

The HMG box of SRY is a calmodulin binding domain

“…CaM transcription factor interactions involving bHLH proteins have been demonstrated in vivo, and shown to inhibit DNA binding [10,25]. The results described in this paper demonstrate similar properties for a different class of nuclear proteins, albeit with lower affinities.…”

“…Many nuclear functions are under CaM control and some transcriptional activators are known to be modulated indirectly through CaM-dependent kinases [8,9]. CaM has more recently been shown to influence transcription by interacting directly with basic helix-loop-helix (bHLH) DNA binding proteins through differential inhibition of the DNA binding activity of these domains [10]. Given the ubiquitous expression of CaM and its binding partner promiscuity, the possibility exists that CaM interacts with other transcription factors to modulate gene expression.…”

B-cell receptor activation inhibits AID expression through calmodulin inhibition of E-proteins

“…The luciferase reporter for E12, 6ϫ ( E5ϩ E2)-luciferase, and the hCMV-␤-gal plasmid for normalization of transfections by dividing the enzymatic activity from the luciferase reporters with the ␤-galactosidase activity are described in ref. 22. The expression vector for mammalian calmodulin (CaM), pcDNAI/amp mCaM, is also described in ref.…”

“…We have shown, however, that Ca 2ϩ -loaded calmodulin can inhibit DNA binding of E2A and other E-proteins by directly binding to the DNA-binding basic sequences in their basic helix-loop-helix domains (22)(23)(24). Thus, we examined the possible participation of E2A in the inhibition of AID gene expression.…”

A Novel Plant Calmodulin-binding Protein with a Kinesin Heavy Chain Motor Domain

“…The effect of this interaction usually results in regulation of enzymatic activity of the binding protein. In animal systems over 20 calmodulin-binding proteins such as protein kinases, a protein phosphatase, a plasma membrane calcium ATPase, an inositol trisphosphate kinase, a nitric oxide synthase, transcription factors, enzymes involved in cyclic nucleotide metabolism, and several cytoskeletal proteins have been characterized (2,16,17). In plants, little is known about the identity and function of calmodulin-binding proteins.…”