Functional heteromerization of HCN1 and HCN2 pacemaker channels

An important step toward understanding the molecular basis of the functiona l diversity of pacemaker currents in spontaneously active cells has been th e identification of a gene family encoding hyperpolarization-activated cycl ic nucleotide-sensitive cation nonselective (HCN) channels. Three of the fo ur gene products that have been expressed so far give rise to pacemaker cha nnels with distinct activation kinetics and are differentially distributed among the brain, with considerable overlap between some isoforms. This rais es the possibility that HCN channels may coassemble to form heteromeric cha nnels in some areas, similar to other K+ channels. In this study, we have p rovided evidence for functional heteromerization of HCN1 and HCN2 channels using a concatenated cDNA construct encoding two connected subunits. We hav e observed that heteromeric channels activate several-fold faster than HCN2 and only a little slower than HCN1. Furthermore, the voltage dependence of activation is more similar to HCN2, whereas the cAMP sensitivity is interm ediate between HCN1 and HCN2. This phenotype shows marked similarity to the current arising from coexpressed HCN1 and HCN2 subunits in oocytes and the native pacemaker current in CA1 pyramidal neurons. We suggest that heterom erization may increase the functional diversity beyond the levels expected from the number of HCN channel genes and their differential distribution.