The pseudogene RPS27AP5 expresses ubiquitin and ribosomal protein variants with potential roles in ribosome function

Pseudogenes, traditionally considered non-functional gene copies, have garnered attention due to emerging evidence of their transcription and translation. Ubiquitin is canonically expressed from UBA52 and RPS27A genes as fusion proteins, with additional polyubiquitin precursors encoded by UBB and UBC. Several pseudogenes of these loci are annotated as non-functional. Here, we report that the RPS27A pseudogene, RPS27AP5, expresses two proteins: a ubiquitin variant (UbP5) and a ribosomal protein variant (S27aP5). These proteins mature through cleavage and exhibit localization and biochemical characteristics similar to their parental counterparts. S27aP5 integrates into ribosomes, and its overexpression leads to an increased 80S monosome fraction. Using affinity purification and polysome profiling, we show that S27aP5-containing ribosomes exhibit altered mRNA associations. The findings suggest that RPS27A, a processed pseudogene, can give rise to a ribosomal protein variant capable of integrating into monosomes and influencing mRNA association aligns with growing evidence that ribosomes may exhibit functional diversity.