The last class of currently available Zn2+ sensors is comprised of
hybrid probes, which have genetically encoded components and
an exogenous cofactor. Two basic designs have been utilized for
Zn2+ probes: the SNAP-tag system and the carbonic anhydrase
platform. The general features of the SNAP-tag system have
been described elsewhere in detail,48b,c,158 and are introduced in
section 2.3.3. The power of this technique is that it allows
small-molecule Zn2+ probes to be targeted to specific cellular
locations. While the Zn2+ sensing and photophysical properties
of the particular probe are unchanged, this strategy does
overcome the issues with ambiguous localization inherent to
small-molecule sensors. The Lippard lab used a BG-conjugated
version of ZP1 and targeted it to the mitochondria and Golgi
with the SNAP-tag system, demonstrating the feasibility of this
approach for targeting small-molecule sensors.50 One potential
challenge of this technique is that modification of a fluorescent
probe with the BG moiety can impact cell permeability, as
demonstrated by the Chang lab for fluorescent probes designed
to sense hydrogen peroxide