Component of the ribosome assembly machinery. Nuclear paralog of the ribosomal protein P0, it binds pre-60S subunits at an early stage of assembly in the nucleolus, and is replaced by P0 in cytoplasmic pre-60S subunits and mature 80S ribosomes. . This is the function of MRTO4 (MRT4 homolog, ribosome maturation factor, Ensembl gene identifier ENSG00000053372).