Lipoproteins cotranslational

A. Cotranslational Lipoprotein Formation in Rough Endoplasmic Reticulum... 

In contrast to the in vitro results, Boren et al. (1992) have shown that lipoprotein biosynthesis occurred cotranslationally in HepG2 cells, that is, while the C-terminal portion of apoB was still being synthesized on the ribosome, the N-terminal portion was already incorporated into a small lipoprotein. 

Fig. 14. Cotranslational assembly of a lipoprotein from the inner leaflet of the ER biiayer and apoB. In this model, translation of the C-terminal portion of apoB proceeds on a membrane-bound ribosome, while translocation and lipoprotein assembly occur on the luminal side of the ER. The N-terminal portion of apoB is believed to be embedded in the inner leaflet of the bilayer, where it nucleates the formation of an oil droplet from the supersaturated ER membranes. As the hydrophobic inner surface of apoB attempts to surround the oil droplet, it bulges into the lumen, as depicted here. On completion of translation, the two ends of apoB become free to meet, which would automatically result in the detachment of the lipoprotein from the bilayer. (Not drawn to scale.)...

Cleavage of the signal sequence in prokaryotes can occur either cotranslationally (Josefsson and Randall, 1981) or posttranslationally (Wu et al., 1983). Cleavage does not appear to be necessary for protein export, as mutants deficient in the cleavage of lipoprotein (Lin etal., 1978), MBP (Ryan et al., 1986a), or the M13 coat protein (Russell and Model, 1981) signal sequences are localized properly. 

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