Nitric Oxide Activates the β2 Subunit of Soluble Guanylyl Cyclase in the Absence of a Second Subunit

Abstract

Previously characterized mammalian soluble guanylyl cyclases form α/β heterodimers that can be activated by the gaseous messenger, nitric oxide, and the novel guanylyl cyclase modulator YC-1. Four mammalian subunits have been cloned named α1, β1, α2, and β2. The α1/β 1 and α2/β1 heterodimeric enzyme isoforms have been rigorously characterized. The role of the β2 subunit has remained elusive. Here we isolate a novel variant of this subunit and show that the β2 subunit does not need to form heterodimers for catalytic activity because enzyme activity can be measured when it is expressed alone in Sf9 cells. In analogy to the β3 subunit recently isolated from the insect Manduca sexta, activity was dependent on the presence of 4 mM free Mn2+. The EC50 values for the NO-donor diethylamine/NO were shifted to the left by 1 order of magnitude as compared with the α1/β1 heterodimeric form. In the presence of the detergent Tween, NO sensitivity of β2 was abolished, but the enzyme could be activated by protoporphyrin IX, indicating removal of a prosthetic heme group and exchange for the heme precursor. We suggest that the β2 subunit is the first mammalian NO-sensitive guanylyl cyclase lacking a heterodimeric structure.