Ribosomal protein P1. This subfamily represents the eukaryotic large ribosomal protein P1. Eukaryotic P1 and P2 are functionally equivalent to the bacterial protein L7/L12, but are not homologous to L7/L12. P1 is located in the L12 stalk, with proteins P2, P0, L11, and 28S rRNA. P1 and P2 are the only proteins in the ribosome to occur as multimers, always appearing as sets of heterodimers. Recent data indicate that eukaryotes have four copies (two heterodimers), while most archaeal species contain six copies of L12p (three homodimers) and bacteria may have four or six copies (two or three homodimers), depending on the species. Experiments using S. cerevisiae P1 and P2 indicate that P1 proteins are positioned more internally with limited reactivity in the C-terminal domains, while P2 proteins seem to be more externally located and are more likely to interact with other cellular components. In lower eukaryotes, P1 and P2 are further subdivided into P1A, P1B, P2A, and P2B, which form P1A/P2B and P1B/P2A heterodimers. Some plant species have a third P-protein, called P3, which is not homologous to P1 and P2. In humans, P1 and P2 are strongly autoimmunogenic. They play a significant role in the etiology and pathogenesis of systemic lupus erythema (SLE). In addition, the ribosome-inactivating protein trichosanthin (TCS) interacts with human P0, P1, and P2, with its primary binding site located in the C-terminal region of P2. TCS inactivates the ribosome by depurinating a specific adenine in the sarcin-ricin loop of 28S rRNA.