Complex coacervation of Mg(ii) phospho-polymethacrylate, a synthetic analog of sandcastle worm adhesive phosphoproteins

Abstract

The highly phosphorylated Pc3 proteins, major components of the sandcastle worm adhesive, are sequestered with Mg as spherical sub-granules within heterogeneous secretory granules in adhesive gland cells. The phase behavior of a synthetic phospho-polymethacrylate analog of the Pc3 phosphoproteins, in the presense of Mg(II), was characterized to determine whether it is chemically possible for the natural adhesive components to be packaged and stored as liquid complex coacervates. Of several multivalent metal salts tested, only MgCl induced complex coacervation of the phospho-copolymer. Complex coacervates formed at Mg/P ratios from 0.5–8, and in [NaCl]s from 0–3 M. At low temperature and pH, the complex coacervates were clear and homogeneous. At higher temperatures and pH, the coacervate phases were translucent. The elastic and viscous moduli initially decreased as temperature increased, but then increased significantly near the temperature boundary between clear and translucent forms. A mechanism is proposed in which relatively weak, ionic strength-independent, outer shell crossbridging of –PO₃²⁻ sidechains by Mg[H₂O]₆²⁺ complex ions is responsible for the clear homogeneous lower viscosity coacervate form. At higher temperature and pH, displacement of inner shell H₂O molecules by phosphate O⁻ ligands creates stronger crossbridges, additional dehydration, and more viscous coacervates. The results demonstrate that Pc3 phosphoproteins can exist as condensed phospho/Mg(II) complex coacervates under conditions expected in the adhesive glands of sandcastle worms in their natural environment. Considering the common regulatory role of phosphorylation and the intracellular abundance of Mg²⁺ it is possible that soft bridging of phosphate groups by Mg[H₂O]_n²⁺ may promote other regulated cellular liquid liquid phase separation phenomena.