Background: There is a pressing need for new sensors that can detect a
variety of analytes, ranging from simple ions to complex compounds an
d even microorganisms. The devices should offer sensitivity, speed, re
versibility and selectivity. Given these criteria, protein pores, remo
deled so that their transmembrane conductances are modulated by the as
sociation of specific analytes, are excellent prospects as components
of biosensors. Results: Structure-based design and a separation method
that employs targeted chemical modification have been used to obtain
a heteromeric form of the bacterial pore-forming protein staphylococca
l a-hemolysin, in which one of the seven subunits contains a binding s
ite for a divalent metal ion, M(II), which serves as a prototypic anal
yte. The single-channel current of the heteromer in planar bilayers is
modulated by nanomolar Zn(II), Other M(II)s modulate the current and
produce characteristic signatures. In addition, heteromers containing
more than one mutant subunit exhibit distinct responses to M(II)s, Hen
ce, a large collection of responsive pores can be generated through su
bunit diversity and combinatorial assembly. Conclusions: Engineered po
res have several advantages as potential sensor elements: sensitivity
is in the nanomolar range; analyte binding is rapid (diffusion limited
in some cases) and reversible; strictly selective binding is not requ
ired because single-channel recordings are rich in information; and fo
r a particular analyte, the dissociation rate constant, the extent of
channel block and the voltage-dependence of these parameters are disti
nguishing, while the frequency of partial channel block reflects the a
nalyte concentration, A single sensor element might, therefore, be use
d to quantitate more than one analyte at once, The approach described
here can be generalized for additional analytes.