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Abstract

STIM1 (a Ca2+ sensor in the endoplasmic reticulum (ER) membrane) and Orai1 (a pore-forming subunit of the Ca2+-release-activated calcium
channel in the plasma membrane) diffuse in the ER membrane and plasma
membrane, respectively. Upon depletion of Ca2+ stores in the ER, STIM1
translocates to the ER-plasma membrane junction and binds Orai1 to
trigger store-operated Ca2+ entry. However, the motion of STIM1 and
Orai1 during this process and its roles to Ca2+ entry is poorly
understood. Here, we report real-time tracking of single STIM1 and Orai1
particles in the ER membrane and plasma membrane in living cells before
and after Ca2+ store depletion. We found that the motion of single STIM1
and Orai1 particles exhibits anomalous diffusion both before and after
store depletion, and their mobility-measured by the radius of gyration
of the trajectories, mean-square displacement, and generalized diffusion
coefficient-decreases drastically after store depletion. We also found
that the measured displacement distribution is non-Gaussian, and the
non-Gaussian parameter drastically increases after store depletion.
Detailed analyses and simulations revealed that single STIM1 and Orai1
particles are confined in the compartmentalized membrane both before and
after store depletion, and the changes in the motion after store
depletion are explained by increased confinement and polydispersity of
STIM1-Orai1 complexes formed at the ER-plasma membrane junctions.
Further simulations showed that this increase in the confinement and
polydispersity after store depletion localizes a rapid increase of Ca2+
influx, which can facilitate the rapid activation of local Ca2+
signaling pathways and the efficient replenishing of Ca2+ store in the
ER in store-operated Ca2+ entry.