In the heart, the hyperpolarization-activated cation current If that plays an important role in initiating spontaneous pacemaking activity is predominantly mediated by the HCN channel subunits HCN2 and HCN4. Here we characterized the subcellular trafficking behavior of HCN4 channels. Nascent HCN4 protein follows the secretory pathway, exiting the endoplasmic reticulum in a COPII-dependent manner. After having reached the cell surface, channels are internalized and sorted into a perinuclear endocytic recycling compartment (ERC) depending on sequence information contained in their cytoplasmic C-termini. From the ERC, channels can recycle back to the plasma membrane in Rab11-positive recycling endosomes. Pharmacological activation of phospholipase D2 (PLD2) promotes transport of HCN4 channels from the ERC to the cell surface whereas disruption of PLD2 signaling by overexpression of a dominant-negative mutant of PLD2 (K758R) reduces surface expression of the channel. Thus, the ERC provides a subcellular storage compartment from which channel protein can be recruited to the cell surface on a fast time scale to dynamically adapt surface expression of HCN4.

(supported by the DFG: GRK 843 and KL-1168/6).