A cell emitting fluorescent signals as a result of attaching specialized proteins to two of its channel-forming IP3Rs (scale bar, 10 µm). Image: National Academy of Sciences USA |
A
research team from the RIKEN Brain Science Institute in Wako, Japan, has
visualized and accurately modeled the molecular changes that open and
close the internal membrane channels for calcium ions within cells. The
ions moving through these channels act as intracellular messengers,
relaying information that regulates the activity of the proteins that
control many critical processes of life and death—from fertilization
through to development, metabolism and, ultimately, death.
Previous
work by the team showed that inositol trisphosphate (IP3) and calcium
ions are involved in regulating channel opening and closing. The
channels are formed from complexes of four IP3 receptors (IP3R) that
bind IP3 and calcium. At low concentrations of calcium ions, channel
opening is stimulated; but at higher levels, it is inhibited. Although
cell biologists have proposed models depicting this process, they had
failed to collect any definitive evidence supporting a particular the
mechanism, until now.
In
live cells, Takayuki Michikawa, Katsuhiko Mikoshiba and their
colleagues attached fluorescent proteins to two of the channel-forming
IP3Rs because these receptors change shape in response to the binding of
IP3 and calcium, and energy flows between this pair of proteins in a
process known as Förster resonance energy transfer (FRET). In a
detectable way, FRET changes the fluorescent light emitted, so the
impact of such links on the conformation of the channel can be studied.
The
researchers found there were at least five binding sites on each IP3R,
one for IP3 and at least four for calcium. Binding IP3 tended to bring
the receptors forming the channel closer together, while calcium tended
to make them relax. But the effects of combining the two were not simply
additive. At a constant level of IP3, they observed an optimum
concentration of calcium that had the most impact on opening the
channel.
From
these results, the researchers proposed a model whereby IP3 and calcium
ions compete with one another—the binding of IP3 prevents calcium
linking to certain sites, and vice versa. High concentrations of calcium
prevent IP3 from binding at all. Further, the researchers proposed two
different types of calcium binding sites: low-affinity sites responsible
for channel activation, and high-affinity sites for inactivation.
“During
the past five years, we have succeeded in visualizing IP3 dynamics and
calcium pump activity,” Michikawa and Mikoshiba say. “In combination
with the model for the calcium release channel described in this study,
we are now ready to understand what happens in living cells during
calcium ion oscillations.”
The corresponding author for this highlight is based at the Laboratory for Developmental Neurobiology, RIKEN Brain Science Institute
