부산대학교 도서관

Regulation of inositol 1,4,5-trisphosphate (IP3) receptors (IP3R) by IP3 and Ca2+ allows them to initiate and regeneratively propagate intracellular Ca2+ signals. The distribution and mobility of IP3R determines the spatial organization of these Ca2+ signals. Until now, there has been no systematic comparison of the distribution and mobility of the three mammalian IP3R subtypes in a uniform background. We used confocal microscopy and fluorescence recovery after photobleaching to define these properties for each IP3R subtype expressed heterologously in COS-7 cells. IP3R1 and 1P3R3 were uniformly distributed within the membranes of the endoplasmic reticulum (ER), but the distribution of 1P3R2 was punctate. The mobile fractions (Mf = 84 ± 2 and 80 ± 2%) and diffusion coefficients (D = 0.018 ± 0.001 and 0.016 ± 0.002 μm²/s) of IP3R1 and 1P3R3 were similar. Other ER membrane proteins (ryanodine receptor type 1 and sarco/endoplasmic reticulum Ca2+-ATPase type 1) and a luminal protein (enhanced GFP with a KDEL retrieval sequence) had similar mobile fractions, suggesting that IP3R1 and 1P3R3 move freely within an ER that is largely, although not entirely, continuous. 1P3R2 was less mobile, but 1P3R2 mobility differed between perinuclear (Mf = 47 ± 4% and D = 0.004 ± 0.00 1 μm²/s) and near-plasma membrane (Mf = 64 ± 6% and D = 0.0 13 ± 0.004 μm²/s) regions, whereas 1P3R3 behaved similarly in both regions. We conclude that IP3R1 and 1P3R3 diffuse freely within a largely continuous ER, but 1P3R2 is more heterogeneously distributed and less mobile, and its mobility differs between regions of the cell. [ABSTRACT FROM AUTHOR]