The proximal N-terminus of IRAG is required for potentiation of HCN4 channels

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Abstract
The inositol triphosphate-associated, ER transmembrane proteins IRAG and LRMP are isoform specific regulators of the hyperpolarization-activated cyclic nucleotide-sensitive isoform 4 (HCN4) channel. LRMP prevents cAMP-dependent potentiation of HCN4, while IRAG mimics the effect of cAMP on the channel. We previously showed that regulation by LRMP requires both the N-terminus of HCN4 and a unique orientation of the HCN4 cAMP transduction center, which is comprised of the N-terminal HCN domain, the C-linker, and the S4-S5 linker. However, it remains unknown if the homologous IRAG requires similar structural features to mimic cAMP-dependent potentiation, or if the site and mechanism of action are different between the two regulators. Using patch clamp electrophysiology, we determined that the initial 43 amino acids of IRAG are necessary and sufficient to confer regulation of HCN4. Similar to LRMP, IRAG also requires a portion of the N-terminus of HCN4 to confer its regulatory effects. Also similar to LRMP, two point mutations in the C-linker region, which are the only sequence differences in that region between HCN4 and the other HCN isoforms, were able to eliminate the effect of IRAG suggesting the unique orientation of the cAMP transduction center in HCN4 is likely important for IRAG function. Taken together, these findings suggest a model whereby IRAG and LRMP interact with the channel in similar regions, although potentially in unique ways, and act on the cAMP transduction center with LRMP inhibiting the coupling of this region to gating and IRAG strengthening it. SUMMARY: The ER transmembrane protein IRAG binds to and potentiates HCN4 channels. This study demonstrates that IRAG regulation of HCN4 requires only the first 43 amino acids of IRAG and involves contributions from the N-terminus and cAMP transduction center of HCN4.

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