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

Abstract

BackgroundThe 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 (Leptoxis, Melanoides, Terebralia).ResultsThe "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 Leptoxis (p50 ~9 mmHg), and a very high affinity in Melanoides (~3 mmHg) and Terebralia (~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.ConclusionsIn cerithioid hemocyanin tridecamers ("mega-hemocyanin") the collar complex of the central decamer is substantially enlarged and modified. The preliminary O2 binding 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.

Cryo-EM Structure of a Molluscan Hemocyanin Suggests Its Allosteric Mechanism

“…At 4.5 Å resolution, our cryo-EM density map reveals essential structural details not seen in any previous studies of the entire native MH complex (Gatsogiannis and Markl, 2009; Gatsogiannis et al, 2007; Meissner, 2007; Leib et al, 2010). The pitches of α-helices (Fig.…”

An Alpine ant’s behavioural polymorphism: monogyny with and without internest aggression inTetramorium alpestre

“…Also the average of pairwise intranest relatedness had no influence on the average aggressive behaviour (Spearman rank correlation) implying that aggression, if occurring at all, seems not to depend on intranest relatedness, at least for this pilot study. As far as we know, peaceful behaviour between non-nestmates of monogynous colonies has been rarely observed in 24 species, for example in Allomerus decemarticulatus, A. octoarticulatus (Grangier et al 2008), Lasius austriacus (Steiner et al 2007 and references therein), L. flavus (Steinmeyer et al 2012), and Monomorium pharaonis (Schmidt et al 2010). In monogynous T. alpestre nests, peaceful behaviour might be explained by two hypotheses: (1) The colonies could stay monogynous on the long run, but workers might avoid aggression as it is time and energy consuming and linked to injury and mortality (Davies & Houston 1984; Cole 1986; Crozier 1987).…”

Cross-bracing uncalibrated nodes in molecular dating improves congruence of fossil and molecular age estimates

“…Here, we test this proposed extension of cross-bracing, using as a test case the hemocyanin gene family of chelicerate arthropods. Hemocyanins constitute the oxygen-transporting metalloproteins of various arthropods and mollusks [18][19][20][21][22][23][24][25]. Arthropod hemocyanins are composed of various subunits, each of which contains two copper moieties that reversibly bind oxygen molecules.…”