Structural Characterization of Eukaryotic GTPase Associated Centre.

摘要

The elongation cycle of protein synthesis is driven by two elongation factors that bind to overlapping sites at the base of the ribosomal stalk. Both factors have limited inherent GTPase activity and they rely on the GTPase associated centre to activate GTP hydrolysis at appropriate times during elongation. In eukaryotes, this region consists of a 58-base 28S ribosomal RNA, the P0(P1/P2)2 pentameric stalk complex and the stalk base protein eL12. Due to the dynamic nature of the ribosomal stalk, this region remains as a missing piece in the high-resolution structural studies of the eukaryotic ribosome. In this work, we have characterized the structural organization of the stalk complex. We have identified the stabilizing interactions within P1/P2 heterodimer and showed that P1/P2 heterodimer is preferred over P2 homodimer due to its higher conformational stability. We have also identified an exposed hydrophobic patch on helix-3 of P1 that is important for anchoring P1/P2 heterodimers to P0 and we have mapped two spine helices on P0 as the binding sites for P1/P2 heteodimer. Based on homology modelling and mutagenesis experiments, we have proposed a new model of the eukaryotic stalk complex where the two heterodimers display a P2/P1:P1/P2 topology on P0. Our model provides an explanation for the difference of GTPase activities contributed by each P-protein and the functional contribution of the hydrophobic loop between the two spine helices of P0. Our model represented the stalk complex in an orientation that is the most effective for recruiting translation factors to their binding sites. As an extension to our studies, we have preliminary data showing direct interaction between eL12 and stalk complex. This is a strong suggestion that eL12 contributes to its functional role by transmitting signal for factor binding and activation through direct interaction with the stalk complex. Our work on the GTPase associated centre has supplemented the structural studies of the eukaryotic ribosome and provided a better picture of how the GTPase associated centre contributes to the high efficiency of protein synthesis.