A signal recognition particle receptor gene from the early-diverging eukaryote, Giardia lamblia

The molecular mechanisms for targeting and translocation of secreted proteins are highly conserved from bacteria to mammalian cells, although the machinery is more complex in higher eukaryotes. To investigate protein transport in the early-diverging eukaryote, Giardia lamblia, we cloned the gene encoding the alpha subunit (SRα) of the signal recognition particle (SRP) receptor. SRα is a small GTPase that functions in SRP-ribosome targeting to the ER. Sequence and phylogenetic analyses showed that SRα from G. lamblia is most homologous to SRα proteins from higher eukaryotes, although it lacks some conserved motifs. Specifically, giardial SRα has an N-terminal extension that enables SRα of higher eukaryotes to interact with a beta subunit that anchors it in the ER membrane. While the C-terminal regions are similar, giardial SRα lacks a prominent 13 amino acid regulatory loop that is characteristic of higher eukaryotic versions. Thus, giardial SRα resembles that of higher eukaryotes, but likely diverged before the advent of the regulatory loop. The 1.8 kb SRα transcript has extremely short untranslated regions (UTRs): a 1-2 nt 5′- and a 9 nt 3′ UTR with the polyadenylation signal overlapping with the stop codon. RT-PCR, Northern and Western analyses showed that SRα is present at relatively constant levels during vegetative growth and encystation, even though there are extensive changes in endomembrane structures and secretory activity during encystation. Immuno-EM showed that SRα localizes to ER-like structures, strengthening the observation of a typical ER in G. lamblia. Unexpectedly, SRα was also found in the lysosome-like peripheral vacuoles, suggesting unusual protein traffic in this early eukaryote. Our results indicate that the eukaryotic type of cotranslational transport appeared early in the evolution of the eukaryotic cell.