Cotranslational Folding Stimulates Programmed Ribosomal Frameshifting in the Alphavirus Structural Polyprotein

Haley R. Harrington; Matthew H. Zimmer; Laura M. Chamness; Veronica Nash; Wesley D. Penn; Thomas F. Miller; Suchetana Mukhopadhyay; Jonathan P. Schlebach

The topological properties of the structural polyprotein described above have implications for the manner in which it is processed at the ER membrane. Our model suggests the cluster of unmodified cysteines in the 6K protein (C786, C787, C789, and C790) reside at a C-terminal portion of TM3 that is projected into the ER lumen (Fig. 3A) and is therefore inaccessible to palmitoylating enzymes. However, these same residues are palmitoylated in the TF protein, 25 which suggests the orientation of TM3 must become inverted upon frameshifting in order to expose them to the cytosolic leaflet. Such an inversion could potentially occur as a consequence of the membrane integration of TM2 (Fig. 3B ), which exhibits a weak propensity to undergo translocon-mediated membrane integration (Fig. 1E) . Furthermore, the efficiency associated with the transloconmediated membrane integration of TM2 (~ 20 %, Fig. 1E ) is comparable to the frequency of -1PRF in the SINV polyprotein (~16%). 22 Taken together, these observations potentially suggest a connection between the formation of a secondary topomer and -1PRF.