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 (SR alpha) of the signal recognition particle (SRP) receptor. SR alpha is a small GTPase that functions in SRP-ribosome targeting 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 has an N-terminal extension that enables SR alpha of higher eukaryotes to interact 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 diverged before the advent of the regulatory loop. The 1.8 kb SR alpha 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 SR alpha is present at relatively constant levels during vegetative growth and encystation. [References: 37]

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