Comparative ESI-MS study of ∼2.2 MDa native hemocyanins from deep-sea and shore crabs: from protein oligomeric state to biotope

Abstract
In the past years, the potential of electrospray ionization mass spectrometry (ESI-MS) for the observation of intact weak interactions, such as non-covalent protein-ligand, protein-protein, protein-DNA complexes, has spread out. The coupling of ESI with time-of-flight (TOF) and quadrupole-time-of-flight (Q-TOF) analyzers has even enabled the detection of larger complexes with molecular weights greatly higher than 200 kDa. In this paper, we report a comparative ESI-MS study on the protein quaternary structure of native hemocyanins (Hc) from crabs living in different biotopes: a shore crab (Carcinus maenas) and two deep-sea crabs (Segonzacia mesatlantica and Bythograea thermydron). Hc is an extracellular blood protein, composed of several protein chains which can associate in large multimers. The goal of this study is to point out that the oligomerization state of native Hcs is biotope-dependent. Depending on the crab, ESI-MS analyses under non-denaturing conditions reveal different oligomeric forms present in equilibrium in solution. Molecular weights up to 2,235 kDa were measured for the associations of 30 subunits of the Bythograea thermydron Hc. Thanks to ESI-MS analyses, it could be concluded for the first time that the oligomerization state of native Hcs is dependent on the crab environment. The investigation of these different non-covalent self-assemblies is very important for the life history of crabs, since they are directly related with different oxygen binding abilities and thus, with their ability to colonize habitats with different oxygen contents.