Lateral diffusion of inositol 1,4,5-trisphosphate receptor type 1 is regulated by actin filaments and 4.1N in neuronal dendrites

Inositol 1,4,5-trisphosphate receptor typel (IP₃R1) plays an important role in neuronal functions; however, the lateral diffusion of IP ₃R1 on the endoplasmic reticulum membrane and its regulation in the living neurons remain unknown. We expressed green fluorescent protein-tagged IP₃R1 in cultured rat hippocampal neurons and observed the lateral diffusion by the fluorescence recovery after photobleaching technique. IP ₃R1 showed lateral diffusion with an effective diffusion constant of ∼0.3 μm²/s. Depletion of actin filaments increased the diffusion constant of IP₃R1, suggesting that the diffusion of IP ₃R1 is regulated negatively through actin filaments. We also found that protein 4.1N, which binds to IP₃R1 and contains an actin-spectrin-binding region, was responsible for this actin regulation of the IP₃R1 diffusion constant. Overexpression of dominant-negative 4.1N and blockade of 4.1N binding to IP₃R1 increased the IP₃R1 diffusion constant. The diffusion of IP₃R type 3 (IP₃R3), one of the isoforms of IP₃Rs lacking the binding ability to 4.1N, was not dependent on actin filaments but became dependent on actin filaments after the addition of a 4.1N-binding sequence. These data suggest that 4.1N serves as a linker protein between IP₃R1 and actin filaments. This actin filament-dependent regulation of IP₃R1 diffusion may be important for the spatiotemporal regulation of intracellular Ca²⁺ signaling.