C-type natriuretic peptide (CNP) activates the transmembrane guanylyl cyclase
natriuretic peptide receptor-B (NPR-B/GC-B), which stimulates cGMP synthesis
and mediates long bone growth, vasorelaxation, and axonal guidance. Genetic
mutations that disrupt normal signaling through this pathway lead to human
skeletal over- and under-growth. Currently, the role of ATP in receptor activation is
controversial. Although ATP is not required for initial activation, it may stabilize the
catalytic domain at longer time points and increase its affinity for GTP. Enzymatic
time course experiments conducted in crude membranes indicated that 1mM ATP
increases guanylyl cyclase activity 2-3 fold when measured at high (1mM) GTP
concentrations but increases activity 10-fold at low (0.1mM) GTP concentrations.
Moreover, receptor activity declines at a greater rate in the absence of ATP than in
its presence at 0.1mM [GTP] versus 1mM [GTP]. We propose that product
inhibition, the binding of purine nucleotides and pyrophosphate to the catalytic
domain, explains the observed receptor deactivation. Additional experiments are
underway to characterize the roles of P-site inhibitors on GC-B activity, which may
provide new insights into the catalytic mechanism of guanylyl cyclase receptors.