Calcium-phosphate bridge is a novel phosphorylation switch that stabilises protein-complexes during HIV assembly

Calcium (Ca ²⁺ ) and phosphate (PO ₄ ^3- ) are fundamental-element and -chemical group in biology. Specifically, the chemistry of both Ca ²⁺ signalling and phosphorylation switch are independent mechanisms regulating a broad spectrum of biological processes. It is, however, not appreciated that a normal function of phospho-mimic amino acids (aspartate/glutamate) is to interact with Ca ²⁺ at the atomic level. Here, we leveraged HIV-Ca ²⁺ biology in primary cells to describe an unknown layer of regulatory processes via Ca ²⁺ -phosphate (PO ₄ ^3- ) bridge to support protein complex formation. We identified novel HIV phosphorylation sites overlapping Ca ²⁺ binding domains through phospho-proteomics. Integrating primary cells, molecular virology, structural biology, biophysical and ultrastructural analyses, we presented multiple examples of Ca ²⁺ -PO ₄ ^3- bridges that support HIV assembly and function. These include Ca ²⁺ -PO ₄ ^3- bridges: (i) stabilising Pr55 ^Gag -Pr160 ^GagPol complex for virus function; (ii) mediating p6 ^Pol dimerization to support virion maturation; and (iii) modulating viral complex formation to package both viral enzymatic- and cellular-proteins. As the convergent enrichment of these signatured calcium-phosphorylation domains occurs across a wide range of viral and cellular proteins, we propose Ca ²⁺ -PO ₄ ^3- bridge to be a general principle for Ca ²⁺ -coordinated phosphorylation switch to regulate biological processes.