The eukaryotic P1 and P2 ribosomal proteins which constitute, with P0, a pentamer forming the lateral stalk of the 60 S ribosomal subunit, exhibit several differences from their prokaryotic equivalents L7 and L12; in particular, P1 does not have the same primary structure as P2 and both of them are phosphorylated, the significance of the latter remaining unclear. Rat liver P1 and P2 were overproduced in Escherichia coli cells and their interaction with elongation factor eEF-2 was studied. Both recombinant proteins were found to be required for the ribosomedependent GTPase activity of eEF-2, with P2 in the phosphorylated form. The surface plasmon resonance technique revealed that, in vitro, both proteins interact specifically with eEF-2, with a higher affinity for P1 (K d = 3.8 Â 10 28 m) than for P2 (K d = 2.2 Â 10 26 m). Phosphorylation resulted in a moderate increase (two-to four-fold) in these affinities. The interaction of both P1 and P2 (phosphorylated or not) with eEF-2 resulted in a conformational change in the factor, revealed by an increase in the accessibility of Glu554 to proteinase Glu-C. This increase was observed in both the presence and absence of GTP and GDP, which themselves produced marked opposite effects on the conformation of eEF-2. Our results suggest that the two proteins P1 and P2 both interact with eEF-2 inducing a conformational transition of the factor, but have acquired some specific properties during evolution.Keywords: acidic ribosomal proteins; ribosomal; elongation factor 2; phosphorylation.P proteins are present in the 60 S ribosomal subunits of all eukaryotic cells. In mammalian cells, there are three different proteins called P0, P1 and P2. P1 and P2 are 12-kDa acidic proteins which exhibit some similarities to and some differences from the prokaryotic ribosomal proteins L7 and L12. Among the similarities are their localization on the large ribosomal subunit forming a lateral stalk, their molecular mass and physicochemical properties, and their requirement for translational factor activity. Thus, the GTPase activity of the eukaryotic elongation factor eEF-2 which catalyses the translocation of peptidyl-tRNA from the A to the P site of the ribosome is dependent on the presence of P1 and P2 on the large ribosomal subunit [1], just as the GTPase activity of the corresponding prokaryotic factor EF-G is dependent on the presence of L7/L12 on this subunit. A direct interaction between L7/L12 and EF-G has been recently visualized directly by cryoelectron microscopy [2]. The differences between P1/P2 and L7/L12 are numerous and interesting to study in terms of evolution. First, sequence homologies between eukaryotic and prokaryotic proteins are not obvious [3]. Secondly, P1 and P2 do not have the same primary structure [3], whereas L7 and L12 do, with the only exception being an N-acetyl group. Thirdly, in contrast with L7 and L12, P1 and P2 are phosphorylated on serine residues when present on the ribosome [4], and this phosphorylation seems to be important for their function, althoug...