The regulatory subunit of PKA-I remains partially structured and undergoes β-aggregation upon thermal denaturation

Background: The regulatory subunit (R) of cAMP-dependent protein kinase (PKA) is a modular flexible protein that responds with large conformational changes to the binding of the effector cAMP. Considering its highly dynamic nature, the protein is rather stable. We studied the thermal denaturation of full-length RIα and a truncated RIα(92-381) that contains the tandem cyclic nucleotide binding (CNB) domains A and B. Methodology/Principal Findings: As revealed by circular dichroism (CD) and differential scanning calorimetry, both RIα proteins contain significant residual structure in the heat-denatured state. As evidenced by CD, the predominantly α-helical spectrum at 25°C with double negative peaks at 209 and 222 nm changes to a spectrum with a single negative peak at 212-216 nm, characteristic of β-structure. A similar α→β transition occurs at higher temperature in the presence of cAMP. Thioflavin T fluorescence and atomic force microscopy studies support the notion that the structural transition is associated with cross-β-intermolecular aggregation and formation of non-fibrillar oligomers. Conclusions/Significance: Thermal denaturation of RIα leads to partial loss of native packing with exposure of aggregation-prone motifs, such as the B' helices in the phosphate-binding cassettes of both CNB domains. The topology of the β-sandwiches in these domains favors inter-molecular β-aggregation, which is suppressed in the ligand-bound states of RIα under physiological conditions. Moreover, our results reveal that the CNB domains persist as structural cores through heat-denaturation. © 2011 Dao et al

Reduced atrial connexin43 expression after pediatric heart surgery

Myocardial dysfunction and arrhythmias may be induced by congenital heart defects, but also be the result of heart surgery with cardiopulmonary bypass (CPB), potentially caused by differential expression of connexin40 (Cx40) and connexin43 (Cx43). In 16 pediatric patients undergoing corrective heart surgery, connexin mRNA expression was studied in volume overloaded (VO group, n=8) and not overloaded (NO group, n=8) right atrial myocardium, excised before and after CPB. Additionally, in eight of these patients ventricular specimens were investigated. The atrial Cx43 expression decreased during CPB, which was restricted to the VO group (p=0.008). In contrast, atrial Cx40 mRNA did not change during CPB. In ventricular myocardium compared to atrial mRNA levels, Cx40 was lower (p=0.006) and Cx43 higher (p=0.017) expressed, without significant change during CPB. This study revealed a significant influence of CPB and the underlying heart defect on Cx43 expression