Alternatively spliced forms of the P180 ribosome receptor differ in their ability to induce the proliferation of rough endoplasmic reticulum

Expression of the canine 180-kDa ribosome receptor p180 in yeast induces the synthesis of RER, and increases the mRNAs of secretory pathway proteins, and protein secretion. To assess whether p180 is a master regulator of cell secretion in mammalian cells, we stably expressed red fluorescent forms of the human p180 variants p180DeltaR (no tandem repeats), p180R (26 repeats), and full-length p180FR (54 repeats) containing different lengths of the tandem repeat ribosome-binding domain in rat pancreatic RINm5F islet beta-cells. All three fluorescent p180 variants localized exclusively to the RER. Cells transfected with p180R were filled with ribosome-studded karmellae, whereas p180DeltaR and p180FR transfectants contained only increased amounts of mostly smooth ER. Unlike in yeast, over-expression of p180R failed to increase the secretory pathway proteins calnexin, SEC61beta, and calreticulin, or ribosome biogenesis. The data suggest that alternative splicing of the p180 tandem repeat domain is a means of regulating the ribosome-binding activity of p180, and potentially the secretory activity of the cell. However, p180 is not a master regulator of mammalian cell secretion as it does not concomitantly trigger the synthesis of protein machinery required to enhance protein synthesis and cell secretion.