Functional characterization of disease-associated mutations in type I cGMP-dependent protein kinase

Cardiovascular diseases are the leading cause of death worldwide, and while mainly associated with behavioral risk factors, they can also be linked to genetic disorders. The cGMP-dependent protein kinase type I (PKG I) plays a pivotal role in the homeostasis of the vasculature by mediating NO/cGMP-dependent relaxation of vascular smooth muscle cells. A recently described genetic risk factor associated with PKG I results in heritable thoracic aortic aneurysms and dissections (TAAD). Although PKG isoforms Iα and Iβ have been studied for the last decades, the molecular mechanisms that drive the differences between both isoforms are still not completely understood. One aim of this thesis was to gain a better understanding of the regulation of PKG I isoforms. Kinase assays employing different peptide substrates highlighted the importance of the autoinhibitory sequence (AS) for kinase activation, basal and stimulated kinase activity, as well as isoform-specific differences in cGMP-dependent activation. Furthermore, the AS critically affects binding kinetics and cGMP affinities in both isoforms. Based on its high-affinity AS, PKG Iβ exhibits transient cGMP-binding, whereas in PKG Iα, with its lower-affinity AS, the high-affinity binding site and the transient low-affinity binding site could be distinguished. The binding pattern was altered in the presence of Mg2+ and ATP indicating a possible role in fast on- and off-switching of PKG I activity in a physiological environment. In the second part of this thesis, two PRKG1 variants of uncertain significance (VUS), associated with aortic aneurysm formation (PKG Iα V219I and PKG Iα G370S), were characterized. V219I, located in the second cyclic nucleotide-binding domain (CNB-B), was shown to cause a cyclic nucleotide (cNMP) sensitization of PKG I. Using SPR and FP-assays, it was demonstrated that this sensitization was based on an increased cNMP affinity. PKG Iα G370S, a mutation located in the glycine-rich loop of the catalytic domain, reduces the KM value of ATP while simultaneously reducing the catalytic turnover of serine and especially threonine substrates. These mutations render the kinase either hyper- (V219I) or hypoactive (G370S) under physiological conditions which suggests a relationship between the mutations and the disease phenotype.