Hemocyanin in mollusks—A molecular survey and new data on hemocyanin genes in Solenogastres and Caudofoveata

Abstract

The most common respiratory protein of mollusks is the blue, copper-containing hemocyanin (van Holde and Miller, 1995). It is not bound to hemocytes but suspended in the hemolymph. Its molecular mass ranges from 3500 10 to 8000 10 Da (dalton) or even more (Herskovits, 1988). These differences in molecular weight are due to the fact that the basic decamers that constitute the barrel-shaped protein may aggregate to didecamers or multidecameric elongated particles (Herskovits, 1988). In cephalopods and chitons (Polyplacophora), there are exclusively decamers, whereas in protobranch bivalves and gastropods the predominantly observed aggregation state is didecamers (Herskovits, 1988; van Holde andMiller, 1995; Lieb andMarkl, 2004; Bergmann et al., 2006, 2007; Gatsogiannis et al., 2007). A typical hemocyanin monomer is composed of eight globular functional units (FU), which are arranged like pearls on a string and termed FU-a to FU-h. An exception are cephalopod hemocyanins that contain only seven FUs. According to molecular clock calculations, the single FUs evolved within the early Precambrian, thus they were present already before the extant molluscan classes derived (Lieb et al., 2000; Lieb and Markl, 2004). Further analyses of hemocyanin-genes from cephalopods, gastropods, a protobranch bivalve, and polyplacophorans (the latter is unpublished data) showed that all hemocyanin genes possess highly conserved phase 1 linker-introns, which separate the individual FU-exons from each other [(FU-a-intron-FU-b-intron-FU-c-intron-FU-d-intron...) Lieb et al., 2001; Altenhein et al., 2002; Bergmann et al., 2006, 2007]. Since these introns are present in all molluscan hemocyanin genes, they seem to be ancient and probably at first used to border a sin-