Abstract
Phycodnaviruses are large dsDNA, algal-infecting viruses that encode many genes with homologs in prokaryotes and eukaryotes. Among the viral gene products are the smallest proteins known to form functional K+ channels. To determine if these viral K+ channels are the product of molecular piracy from their hosts, we compared the sequences of the K+ channel pore modules from seven phycodnaviruses to the K+ channels from Chlorella variabilis and Ectocarpus siliculosus, whose genomes have recently been sequenced. C. variabilis is the host for two of the viruses PBCV-1 and NY-2A and E. siliculosus is the host for the virus EsV-1. Systematic phylogenetic analyses consistently indicate that the viral K+ channels are not related to any lineage of the host channel homologs and that they are more closely related to each other than to their host homologs. A consensus sequence of the viral channels resembles a protein of unknown function from a proteobacterium. However, the bacterial protein lacks the consensus motif of all K+ channels and it does not form a functional channel in yeast, suggesting that the viral channels did not come from a proteobacterium. Collectively, our results indicate that the viruses did not acquire their K+ channel-encoding genes from their current algal hosts by gene transfer; thus alternative explanations are required. One possibility is that the viral genes arose from ancient organisms, which served as their hosts before the viruses developed their current host specificity. Alternatively the viral proteins could be the origin of K+ channels in algae and perhaps even all cellular organisms.
Highlights
In recent years several virus-encoded proteins with ion channel activity have been described [1,2,3,4]
A genomic analysis of 40 virus isolates from a single species, all of which replicate in C. variabilis, revealed that the channel proteins differed by as many as 16 amino acids from the reference channel KcvPBCV-1 [19,35]
Our analyses indicate that viral-encoded K+ channel proteins do not have a close phylogenetic relationship with their host-encoded K+ channel proteins
Summary
In recent years several virus-encoded proteins with ion channel activity have been described [1,2,3,4] These proteins show few common features at the sequence level, except that most of them are short, approximately 100 amino acid residues, and their membrane-spanning domains are predicted to be a-helices [1]. The majority of these viral-encoded channel proteins have no recognizable sequence similarity to bacterial or eukaryotic proteins. A different situation occurs with ion channel proteins encoded by the virus family Phycodnaviridae These viruses, which infect algae [6], have gene products with the structural and functional hallmarks of eukaryotic and prokaryotic K+ channels [4]. Like complex eukaryotic channels this channel functions as a tetramer [8,9]
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