We recently identified two ActRIIB paralogs in the marine fish gilthead sea bream Sparus aurata Linnaeus, 1758: ActRIIB-1 and ActRIIB-2 (with two alleles: 2a and 2b) (Funkenstein et al., 2012).
For comparative studies of the biological activities of the two ActRIIB paralogs, clone #1 of saActRIIB-l-ECD/pPICZ[alpha]A was expressed and purified essentially as described above for saActRIIB-2a-ECD/pPICZaA.
However, both ECDs were effective in inhibiting MSTN and activin A, which suggests that both ActRIIB paralogs act in a similar way to mammalian peptides.
In the present study, we describe the expression, purification, and biological activity of the soluble form of the second paralog of ActRIIB found in fish: ActRIIB-2a-ECD.
Most likely, they sequester MSTN and activin away from full-length, membrane-bound ActRIIB. Earlier studies in mammals, in which direct binding assays of soluble ActRII and ActRIIB to radiolabeled activins and inhibins were tested, revealed that both receptors bind activin A with high affinity (del Re et al., 2004).
A detailed kinetic characterization of soluble ActRIIB binding to several low- and high-affinity ligands demonstrated that both MSTN and GDF-11 bind ActRIIB-ECD with affinities comparable to those of activin A (Sako et al., 2010).
Number and position of amino acid residues in mouse ActRIIB, shown earlier to be directly involved in binding to activin (Gray et al., 2000); Thompson et al., 2003), are also conserved in saActRIIB-l-ECD and saActRIIB-2a-ECD.