Ribosomal protein P1, P2, and L12p. Ribosomal proteins P1 and P2 are the eukaryotic proteins that are functionally equivalent to bacterial L7/L12. L12p is the archaeal homolog. Unlike other ribosomal proteins, the archaeal L12p and eukaryotic P1 and P2 do not share sequence similarity with their bacterial counterparts. They are part of the ribosomal stalk (called the L7/L12 stalk in bacteria), along with 28S rRNA and the proteins L11 and P0 in eukaryotes (23S rRNA, L11, and L10e in archaea). In bacterial ribosomes, L7/L12 homodimers bind the extended C-terminal helix of L10 to anchor the L7/L12 molecules to the ribosome. Eukaryotic P1/P2 heterodimers and archaeal L12p homodimers are believed to bind the L10 equivalent proteins, eukaryotic P0 and archaeal L10e, in a similar fashion. P1 and P2 (L12p, L7/L12) are the only proteins in the ribosome to occur as multimers, always appearing as sets of dimers. Recent data indicate that most archaeal species contain six copies of L12p (three homodimers), while eukaryotes have two copies each of P1 and P2 (two heterodimers). Bacteria may have four or six copies (two or three homodimers), depending on the species. As in bacteria, the stalk is crucial for binding of initiation, elongation, and release factors in eukaryotes and archaea.