Key result
The Giardiavirus internal ribosome entry site initiates translation through a unique mechanism that does not require a pre-initiation complex but involves binding to specific trans-acting protein factors prior to ribosomal recruitment.
The GLV IRES utilizes a unique translation initiation mechanism that does not require a pre-initiation complex but involves specific trans-acting factors like IBP1, La, and SRp20.
Giardiavirus (GLV) utilizes an internal ribosome entry site (IRES) for translation initiation in the early branching eukaryote Giardia lamblia. Unlike most of the viral IRESs among higher eukaryotes, which localize primarily within the 5'-untranslated region (UTR), the GLV IRES comprises 253 nts of 5'UTR and the initial 264 nts in the open-reading-frame (ORF). To test if GLV IRES also functions in higher eukaryotic systems, we examined it in rabbit reticulocyte lysate (RRL) and found that it functions much less efficiently than the IRES from the Encephalomyocarditis virus (EMCV) or Cricket paralysis virus (CrPV). In contrast, both EMCV-IRES and CrPV-IRESs were inactive in transfected Giardia cells. Structure-function analysis indicated that only the stem-loop U5 from the 5'UTR and the stem-loop I plus the downstream box (Dbox) from the ORF of GLV IRES are required for limited IRES function in RRL. Edeine, a translation initiation inhibitor, did not significantly affect the function of GLV IRES in either RRL or Giardia, indicating that a pre-initiation complex is not required for GLV IRES-mediated translation initiation. However, the small ribosomal subunit purified from Giardia did not bind to GLV IRES, indicating that additional protein factors may be necessary. A member of the helicase family IBP1 and two known viral IRES binding proteins La autoantigen and SRp20 have been identified in Giardia that bind to GLV IRES in vitro. These three proteins could be involved in facilitating small ribosome recruitment for initiating translation.
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Garlapati et al. (2009) studied Giardiavirus translation initiation (Basic Science). Giardiavirus Internal Ribosome Entry Site (GLV IRES) vs. Control transcripts, EMCV IRES, CrPV IRES was evaluated on Translation initiation efficiency and mechanism. The Giardiavirus internal ribosome entry site initiates translation through a unique mechanism that does not require a pre-initiation complex but involves binding to specific trans-acting protein factors prior to ribosomal recruitment.
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