Calmodulin is Critical for Folding of the Kv7.2 Calcium-Responsive Domain As the Nascent Peptide Exits the Ribosome

Traditionally, protein folding has been analyzed through in vitro unfolding and refolding approaches, which fall short of emulating the co-translational folding events that occur within cellular environments. Consequently, our understanding of co-translational folding, particularly in ion channels and other proteins, remains limited. Our research has illuminated the pivotal role of calmodulin (CaM) in orchestrating the vectorial folding process of the calcium-responsive domain (CRD) within the Kv7.2 channel. We employ force profile analysis (FPA) to gauge the force applied to the nascent chain during the initial folding stages. In this study, we delineate the force profile of the CRD during translation, revealing that CaM is indispensable for instigating early folding events in the three pivotal helices: the IQ site of helix A, helix TW, and helix B. In a surprising turn, we observe that CaM also exerts influence on the non-target helices C and D, which constitute the subunit interaction domain of this channel. Furthermore, we have discovered that CaM assists in facilitating the co-translational folding process of the SK4 channel. This investigation sheds fresh light on the intricate role played by CaM in facilitating the co-translational folding mechanism of the Kv7.2 channel's C-terminal region and suggests potential involvement of CaM and calcium in the co-translational folding of other select proteins.