This family of ribosomal proteins consists mainly of the 40S ribosomal protein S27a which is synthesised as a C-terminal extension of ubiquitin (CEP). The S27a domain compromises the C-terminal half of the protein. The synthesis of ribosomal proteins as extensions of ubiquitin promotes their incorporation into nascent ribosomes by a transient metabolic stabilisation and is required for efficient ribosome biogenesis [3]. The ribosomal extension protein S27a contains a basic region that is proposed to form a zinc finger; its fusion gene is proposed as a mechanism to maintain a fixed ratio between ubiquitin necessary for degrading proteins and ribosomes a source of proteins [2].

1: Characterization and expression of two chicken cDNAs encoding ubiquitin fused to ribosomal proteins of 52 and 80 amino acids. Mezquita J, Pau M, Mezquita C; Gene 1997;195:313-319. PMID:9305777

2: Identification of the long ubiquitin extension as ribosomal protein S27a. Redman KL, Rechsteiner M; Nature 1989;338:438-440. PMID:2538756

3: The tails of ubiquitin precursors are ribosomal proteins whose fusion to ubiquitin facilitates ribosome biogenesis. Finley D, Bartel B, Varshavsky A; Nature 1989;338:394-401. PMID:2538753