Molluscan mega-hemocyanin: an ancient oxygen carrier tuned by a ~550 kDa polypeptide

<p>Abstract</p> <p>Background</p> <p>The allosteric respiratory protein hemocyanin occurs in gastropods as tubular di-, tri- and multimers of a 35 × 18 nm, ring-like decamer with a collar complex at one opening. The decamer comprises five subunit dimers. The subunit, a 400 kDa polypeptide, is a concatenation of eight paralogous functional units. Their exact topology within the quaternary structure has recently been solved by 3D electron microscopy, providing a molecular model of an entire didecamer (two conjoined decamers). Here we study keyhole limpet hemocyanin (KLH2) tridecamers to unravel the exact association mode of the third decamer. Moreover, we introduce and describe a more complex type of hemocyanin tridecamer discovered in fresh/brackish-water cerithioid snails (<it>Leptoxis</it>, <it>Melanoides</it>, <it>Terebralia</it>).</p> <p>Results</p> <p>The "typical" KLH2 tridecamer is partially hollow, whereas the cerithioid tridecamer is almost completely filled with material; it was therefore termed "mega-hemocyanin". In both types, the staggering angle between adjoining decamers is 36°. The cerithioid tridecamer comprises two typical decamers based on the canonical 400 kDa subunit, flanking a central "mega-decamer" composed of ten unique ~550 kDa subunits. The additional ~150 kDa per subunit substantially enlarge the internal collar complex. Preliminary oxygen binding measurements indicate a moderate hemocyanin oxygen affinity in <it>Leptoxis </it>(p50 ~9 mmHg), and a very high affinity in <it>Melanoides </it>(~3 mmHg) and <it>Terebralia </it>(~2 mmHg). Species-specific and individual variation in the proportions of the two subunit types was also observed, leading to differences in the oligomeric states found in the hemolymph.</p> <p>Conclusions</p> <p>In cerithioid hemocyanin tridecamers ("mega-hemocyanin") the collar complex of the central decamer is substantially enlarged and modified. The preliminary O₂binding curves indicate that there are species-specific functional differences in the cerithioid mega-hemocyanins which might reflect different physiological tolerances of these gill-breathing animals. The observed differential expression of the two subunit types of mega-hemocyanin might allow individual respiratory acclimatization. We hypothesize that mega-hemocyanin is a key character supporting the adaptive radiation and invasive capacity of cerithioid snails.</p

The complete mitochondrial genome of Harpovoluta charcoti (Gastropoda: Neogastropoda: Volutidae)

We report the complete mitochondrial genome sequence of Harpovoluta charcoti, a circum-Antarctic species of volutid gastropod inhabiting bathyal soft bottom substrates. The mitogenome is 15,487 bp in length, has a base composition of A (28.3%), T (37.3%), C (16.0%) and G (18.4%), and contains 13 protein-coding genes, two ribosomal RNA genes, and 22 transfer RNA genes. Gene order and strand orientation are the same as in other non-toxoglossan neogastropods. Phylogenetic analyses support the monophyly of Volutidae, but not of the subfamilies Amoriinae or Cymbiinae as currently circumscribed

The complete mitochondrial genome of Costapexbaldwinae (Gastropoda: Neogastropoda:Turbinelloidea: Costellariidae) from the CaribbeanDeep-Sea

We report the complete mitochondrial genome sequence ofCostapex baldwinae, a Caribbean representative of a predominantly Indo-Pacific genus of gastropods that occurs on sunken wood at bathyaldepths. The mitogenome is 15,321bp in length and has a base composition of 29.2% A, 41.8% T,12.0% C and 17.0% G. It contains 13 protein-coding, two ribosomal RNA, and 22 tRNA genes with thesame gene order and strand orientation as other non-toxoglossan neogastropods. Phylogenetic analy-ses indicate that the superfamily Turbinelloidea, represented by this species, diverged early within theNeogastropod radiation, forming the sister group to a clade that includes five of the seven presentlyrecognized superfamilies.The contribution of J. Uribe was supported by the Peter Buck Postdoctoral Fellowship Program from the Smithsonian Institution [2017–2019] and Atracción Talento de la Comunidad de Madrid Fellowship Program [REFF 2019-T2/AMB-13166]. The contribution of A.Fedosov was supported by the Russian Science Foundation grant [19-74-10020].Peer reviewe