Calmodulin assists during co-translational folding of the K_V7.2 channel calcium responsive domain

In vivo, the majority of nascent protein chains begin folding during translation in order to reach their native structure. While the importance of co-translational folding has become increasingly clear, the specific mechanisms underlying the coordination between the ribosome, the nascent chain and interacting partners are still uncertain. Here, we show that calmodulin (CaM) plays a prominent role at discrete steps of the co-translational folding pathway of the calcium responsive domain (CRD) of the human neuronal K_V7.2 ion channel, providing grounds for the proposal of a likely folding pathway. By combining force profile analysis and single-molecule force spectroscopy techniques, we found that CaM, in a calcium-dependent manner, affects early folding events involving three key α-helices in the CRD. In addition, this study suggests that CaM at early stages participates in the formation of metastable helical hairpins, as part of the co-translational folding pathway. These findings expand on the role of CaM as a key regulator of co-translational folding.