The soluble state of the HIV-1 Vpu protein forms a complex with Ca²⁺-calmodulin

The HIV-1 Vpu membrane protein is crucial to the virus lifecycle. Our recent studies revealed soluble Vpu oligomers, prompting further investigation into their interactions with cellular proteins. Notably, Vpu may form a complex with calmodulin (CaM) due to its putative CaM-binding motif; however, experimental proof of this association is unavailable. Here, we present definitive experimental evidence that the soluble Vpu complex interacts in vitro with calcium-bound CaM (Ca²⁺-CaM), its active form. Using double electron-electron resonance (DEER) spectroscopy and protein spin labeling, we detected the formation of a soluble Vpu-Ca²⁺-CaM complex. Both the full-length (FL) and truncated C-terminal regions of Vpu bind Ca²⁺-CaM. DEER experiments on a spin-labeled CaM cysteine mutant S39C/A103C revealed that, upon association with Vpu, Ca²⁺-CaM undergoes a transition from an open to a more closed conformation, consistent with previous reports of Ca²⁺-CaM interactions with other proteins. Furthermore, we observed that the binding of Vpu to Ca²⁺-CaM leads to dissociation of soluble Vpu oligomers, as evidenced by a reduction in DEER modulation depth for FL Vpu spin-labeled at residue L42C. FRET analysis with a fluorescently labeled C-terminal cysteine mutant of Vpu confirmed this result. Like FL Vpu, the Vpu C-terminal region forms soluble homooligomers that dissociate upon binding to Ca²⁺-CaM. Collectively, our results suggest that soluble Vpu and Ca²⁺-CaM form an equimolar complex. DEER analysis of the Vpu C-terminal region spin-labeled at residues Q36C/I61C demonstrated that Vpu undergoes significant conformational changes to facilitate Ca²⁺-CaM binding. These findings could be relevant to Vpu-CaM interactions under physiological conditions.