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

Calcium (Ca2+) and phosphate (PO43-) are fundamental-element and -chemical group in biology. Specifically, the chemistry of both Ca2+ 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 Ca2+ at the atomic level. Here, we leveraged HIV-Ca2+ biology in primary cells to describe an unknown layer of regulatory processes via Ca2+-phosphate (PO43-) bridge to support protein complex formation. We identified novel HIV phosphorylation sites overlapping Ca2+ binding domains through phospho-proteomics. Integrating primary cells, molecular virology, structural biology, biophysical and ultrastructural analyses, we presented multiple examples of Ca2+-PO43- bridges that support HIV assembly and function. These include Ca2+-PO43- bridges: (i) stabilising Pr55Gag-Pr160GagPol complex for virus function; (ii) mediating p6Pol 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 Ca2+-PO43- bridge to be a general principle for Ca2+-coordinated phosphorylation switch to regulate biological processes.