Modulation of Guanylate Cyclase Activating Protein 1 (GCAP1) Dimeric Assembly by Ca2+ or Mg2+: Hints to Understand Protein Activity.

Francesco Bonì,Alberto Barbiroli,Mario Milani,Eloise Mastrangelo,Daniele Dell’Orco,Valerio Marino,Carlo Bidoia

doi:10.3390/biom10101408

Abstract

The guanylyl cyclase-activating protein 1, GCAP1, activates or inhibits retinal guanylyl cyclase (retGC) depending on cellular Ca2+ concentrations. Several point mutations of GCAP1 have been associated with impaired calcium sensitivity that eventually triggers progressive retinal degeneration. In this work, we demonstrate that the recombinant human protein presents a highly dynamic monomer-dimer equilibrium, whose dissociation constant is influenced by salt concentration and, more importantly, by protein binding to Ca2+ or Mg2+. Based on small-angle X-ray scattering data, protein-protein docking, and molecular dynamics simulations we propose two novel three-dimensional models of Ca2+-bound GCAP1 dimer. The different propensity of human GCAP1 to dimerize suggests structural differences induced by cation binding potentially involved in the regulation of retGC activity.

Highlights

In photoreceptor cells, light photons are transformed into electric signals by the coordinate interplay of a complex network of proteins, regulated by different chemical messengers [1].Guanylyl cyclase-activating protein 1 (GCAP1) belongs to the calmodulin superfamily and hosts 4 EF-hand motifs (EF1–EF4)
The hydrophobic contacts at the GCAP1 dimer interface have been proven to be essential for both its dimerization and for the activation of RetGC, which is itself a dimer, suggesting that the GCAP1 dimer may bind to the cyclase target to form a 2:2 complex, whose allosteric regulation of catalytic activity may involve quaternary structural changes in a protein-protein complex [16]
In the absence of bRetGC, the dimeric nature of bGCAP1 in the photoreceptor inner segment may prevent its diffusion into the outer segment