Role of the Kinase Domain in TRPM7 Channel Activity and Function

Abstract

Transient Receptor Potential Melastatin 7 is an integral membrane protein comprising a non-specific cation channel, a cytosolic C-terminal kinase and a functionally undefined cytosolic N-terminal domain. TRPM7 is ‘ubiquitously’ expressed, playing vital roles in cell survival, proliferation and differentiation. Proliferating cells generally exhibit larger TRPM7 currents than non-dividing, terminally differentiated cells. Channel properties are well described and are thought to be responsible for the Mg2+/Ca2+ influx supporting cell biological functions. The channel exhibits strong rectification in the presence of extracellular or cytosolic Mg2+. Kinase functionality is less well understood other than that channel properties are unaltered between wild type (WT) and kinase-inactive mutants. Recent work shows that the kinase domain can be cleaved from the channel in vivo [Krapivinsky et al. Cell. 157 (2014) 1061]. The ectopically expressed kinase domain localizes to the nucleoplasm and the kinase-deleted channel (ΔK) exhibits a possible length-dependent range of activities relative to WT. Here we addressed the role of the kinase domain in regulating channel activation. We confirmed the nuclear expression of ectopically expressed murine TRPM7 kinase (residues 1511-1863) and a similar localization of endogenous TRPM7 in proliferating liver cells (WIF-B) suggesting that these cells express the cleaved kinase. Conversely non-dividing hepatocytes are labelled in the nuclear envelop indicating that TRPM7 is mainly intact. Fractional currents (I/Imax) in HEK293 cells at break-in are significantly lower in ΔK (0.18±0.08) compared with WT (0.47±0.12) expressors. Mean activation time constants are 2.5 times lower in ΔK versus WT cells. Removal of the kinase domain increases sensitivity to both extra- and intracellular Mg2+. Endogenous currents in WIF-B cells reflect a hybrid of these properties. We conclude that a complex dynamic exists between subcellular compartmentalization, proteolysis and the cellular role(s) of TRPM7.