Accessory subunit of hyperpolarization-activated cyclic nucleotide-gated (HCN) channels, regulating their cell-surface expression and cyclic nucleotide dependence. Interacts with RAB8B. Forms an obligate 4:4 complex with HCN2 (By similarity). May interact with the C-terminal PTS1-type tripeptide peroxisomal targeting signal (SKL-type); the relevance of such interaction is however unclear (PubMed:11463335). Interacts with HCN3 (By similarity). Some fraction is membrane associated via its interaction with RAB8B. Mainly expressed in brain. Also expressed in pancreas, testis and pituitary. Belongs to the peroxisomal targeting signal receptor family.