Rj. Pease et al., STUDIES ON THE TRANSLOCATION OF THE AMINO-TERMINUS OF APOLIPOPROTEIN-B INTO THE ENDOPLASMIC-RETICULUM, The Journal of biological chemistry, 270(13), 1995, pp. 7261-7271
Apolipoprotein (apo) B is either co-translationally assembled into lip
oproteins, or becomes associated with the membrane of the endoplasmic
reticulum (ER) and is subsequently degraded, It has been proposed that
apoB undergoes a novel process of translocation which generates cytop
lasmically exposed apoB in the ER of hepatic and non-hepatic cells. Tr
ansmembrane forms of apoB can also be generated by in vitro translatio
n (Chuck, S. L., and Lingappa, V. R. (1992) Cell 68, 9-21), which migh
t explain the origin of untranslocated apoB in vivo. Here we have inve
stigated a protocol which generates transmembrane forms of apoB during
in vitro translation of truncated RNA transcripts, We observe that ap
oB can become transmembrane at sites of ribosome pausing and be held i
n this configuration by persistence of tRNA on the peptide chains. Rib
osome pausing also occurs at these same sites in the absence of accept
or microsomes. Transmembrane topology can be generated at sites of rib
osome pausing in a cytosolic protein, sea urchin cyclin when fused to
a signal sequence, Mapping of the ribosome pause sites in apoB and in
cyclin revealed no amino acid sequence homology, Chimeric constructs w
ith engineered downstream glycosylation sites showed no evidence that
ribosome pause sequences affect translocation of transcripts with term
ination codons. Transmembrane forms of apoB and cyclin were not genera
ted during translocation into the ER in transfected COS cells.