Channel-kinase TRPM7/ChaK1 is a member of a recently discovered family of protein kinases called ␣-kinases that display no sequence homology to conventional protein kinases. It is an unusual bifunctional protein that contains an ␣-kinase domain fused to an ion channel. The TRPM7/ChaK1 channel has been characterized using electrophysiological techniques, and recent evidence suggests that it may play a key role in the regulation of magnesium homeostasis. However, little is known about its protein kinase activity. To characterize the kinase activity of TRPM7/ChaK1, we expressed the kinase catalytic domain in bacteria. ChaK1-cat is able to undergo autophosphorylation and to phosphorylate myelin basic protein and histone H3 on serine and threonine residues. The kinase is specific for ATP and cannot use GTP as a substrate. ChaK1-cat is insensitive to staurosporine (up to 0.1 mM) but can be inhibited by rottlerin. Because the kinase domain is physically linked to an ion channel, we investigated the effect of ions on ChaK1-cat activity. The kinase requires Mg 2؉ (optimum at 4 -10 mM) or Mn 2؉ (optimum at 3-5 mM), with activity in the presence of Mn 2؉ being 2 orders of magnitude higher than in the presence of Mg 2؉ . Zn 2؉ and Co 2؉ inhibited ChaK1-cat kinase activity. Ca 2؉ at concentrations up to 1 mM did not affect kinase activity. Considering intracellular ion concentrations, our results suggest that, among divalent metal ions, only Mg 2؉ can directly modulate TRPM7/ChaK1 kinase activity in vivo.A new family of protein kinases that do not display sequence homology to conventional eukaryotic protein kinases has been recently identified (1, 2). When mammalian and Caenorhabditis elegans elongation factor-2 kinases (eEF-2 1 kinases) were cloned, it was found that they do not display sequence homology to any conventional eukaryotic protein kinase (1). However, their catalytic domains appeared to be homologous to the catalytic domain of myosin heavy chain kinase A from Dictyostelium (3-5). Two more protein kinases with the same type of catalytic domain have been subsequently identified in Dictyostelium and have been called myosin heavy chain kinases B and C (6, 7). This new family of protein kinases was named ␣-kinase, because the existing evidence suggests that these protein kinases phosphorylate amino acids located within ␣-helices (2). This is different from conventional protein kinases that phosphorylate amino acids located within loops, turns, or regions with irregular structure (8). The ␣-kinase catalytic domain is characterized by several conserved motifs, which are different from the distinguishing sequence motifs that are found in conventional protein kinases (2). Surprisingly, the recently determined three-dimensional structure of the ␣-kinase catalytic domain revealed that, despite the lack of sequence homology, ␣-kinases have a fold that is very similar to conventional eukaryotic protein kinases (9).Five more human proteins with the ␣-kinase domain have been identified and cloned (2, 10, 11). Unexpectedly, it ...