Accessory subunit of hyperpolarization-activated cyclic nucleotide-gated (HCN) channels, regulating their cell-surface expression and cyclic nucleotide dependence. Forms an obligate 4:4 complex with HCN2 (By similarity). Interacts with RAB8B (PubMed:11278749). Interacts with HCN3 (By similarity). Some fraction is membrane associated via its interaction with RAB8B. Brain specific. Belongs to the peroxisomal targeting signal receptor family.