Accessory subunit of hyperpolarization-activated cyclic nucleotide-gated (HCN) channels, regulating their cell-surface expression and cyclic nucleotide dependence. Interacts with RAB8B (By similarity). Forms an obligate 4:4 complex with HCN2 (PubMed:22550182). Interacts with HCN3 (Ref.5). Some fraction is membrane associated via its interaction with RAB8B. Belongs to the peroxisomal targeting signal receptor family.