Neuronal calcium sensors emerge as novel adenylyl cyclase 8 regulators

Adenylyl cyclase 8 (AC8) is a Ca ²⁺ -sensitive membrane adenylyl cyclase highly expressed in the central nervous system. AC8-mediated intracellular cyclic adenosine monophosphate (cAMP) accumulation shapes synaptic function, plasticity, and memory formation and is tightly controlled by intracellular Ca ²⁺ and G protein–coupled receptor signaling. Although it is well known that the effects of Ca ²⁺ on AC8 activity are directly mediated by calmodulin, it has remained unclear until now whether other Ca ²⁺ -binding proteins also regulate AC8 function. Here, we identify the neuronal calcium sensor hippocalcin-like protein 1 (HPCAL1) as a direct interaction partner of AC8. Using biochemistry, structural proteomics, integrative modeling, cryo-EM and cell-based FRET approaches, we show that HPCAL1 associates with AC8 in a Ca ²⁺ -dependent manner. HPCAL1 shows weak positive modulation of AC8 activity in vitro , with sub micromolar affinity. HPCAL1 interacts with flexible regulatory regions of AC8, including the C1b domain and the N- and C-terminal regions. Interestingly, other members of the neuronal calcium sensor family can bind AC8 via the same sites. Together, our study reveals a previously unrecognized mode of Ca ²⁺ -dependent AC8 recognition by a group of neuronal calcium sensor proteins, which may be relevant to context-dependent regulation of cAMP signaling.