Expression of distinct ERG proteins in rat, mouse, and human heart - Relation to functional I-Kr channels

One form of inherited long QT syndrome, LQT2, results from mutations in HER G1, the human ether-a-go-go-related gene, which encodes a voltage-gated Kchannel alpha subunit. Heterologous expression of HERG1 gives rise to K+ cu rrents that are similar (but not identical) to the rapid component of delay ed rectification, I-Kr in cardiac myocytes, In addition, N-terminal splice variants of HERG1 and MERG1 (mouse ERG1) referred to as HERG1b and NIERG1b have been cloned and suggested to play roles in the generation of functiona l I-Kr channels. In the experiments here, antibodies generated against HERG 1 were used to examine ERG1 protein expression in heart and in brain. In We stern blots of extracts of QT-6 cells expressing HERG1, MERG1, dr RERG1 (ra t ERG1) probed with antibodies targeted against the C terminus of HERG1, a single 155-kDa protein is identified, whereas a 95-kDa band is evident in b lots of extracts from cells expressing MERG1b or HEBG1b. In immunoblots of fractionated rat (and mouse) brain and heart membrane proteins, however, tw o prominent high molecular mass proteins of 165 and 205 kDa were detected. Following treatment with glycopeptidase F, the 165- and 205-kDa proteins we re replaced by two new bands at 175 and 130 kDa, suggesting that ERG1 is di fferentially glycosylated in rat/mouse brain and heart. In human heart, a s ingle HERG1 protein with an apparent molecular mass of 145 kDa is evident. In rats, ERG1 protein (and I-Kr) expression is higher in atria than ventric les, whereas in humans, HERG1 expression is higher in ventricular, than atr ial, tissue. Taken together, these results suggest that the N-terminal alte rnatively spliced variants of ERG1 (i.e. ERG1b) are not expressed at the pr otein level in rat, mouse, or human heart and that these variants do not, t herefore, play roles in the generation of functional cardiac I-Kr channels.