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

The molecular mechanisms for targeting and translocation of secreted protei ns are highly conserved from bacteria to mammalian cells, although the mach inery is more complex in higher eukaryotes. To investigate protein transpor t in the early-diverging eukaryote, Giardia lamblia, we cloned the gene enc oding the alpha subunit (SR alpha) of the signal recognition particle (SRP) receptor. SR alpha is a small GTPase that functions in SRP-ribosome target ing to the ER. Sequence and phylogenetic analyses showed that SR alpha from G. lamblia is most homologous to SR alpha proteins from higher eukaryotes, although it lacks some conserved motifs. Specifically, giardial SR alpha h as an N-terminal extension that enables SR alpha of higher eukaryotes to in teract with a beta subunit that anchors it in the ER membrane. While the C- terminal regions are similar, giardial SR alpha lacks a prominent 13 amino acid regulatory loop that is characteristic of higher eukaryotic versions. Thus, giardial SR alpha resembles that of higher eukaryotes, but likely div erged before the advent of the regulatory loop. The 1.8 kb SR alpha transcr ipt has extremely short untranslated regions (UTRs): a 1-2 nt 5'- and a 9 n t 3' UTR with the polyadenylation signal overlapping with the stop codon. R T-PCR, Northern and Western analyses showed that SR alpha is present at rel atively constant levels during vegetative growth and encystation.