The GTP binding motif: variations on a theme.

GTP binding proteins (G-proteins) have wide-ranging functions in biology, being involved in cell proliferation, signal transduction, protein synthesis, and protein targeting. Common to their functioning is that they are active in the GTP-bound form and inactive in the GDP-bound form. The protein synthesis elongation factor EF-Tu was the first G-protein whose nucleotide binding domain was solved structurally by X-ray crystallography to yield a structural definition of the GDP-bound form, but a still increasing number of new structures of G-proteins are appearing in the literature, in both GDP and GTP bound forms. A common structural core for nucleotide binding is present in all these structures, and this core has long been known to include common consensus sequence elements involved in binding of the nucleotide. Nevertheless, subtle changes in the common sequences reflect functional differences. Therefore, it becomes increasingly important to focus on how these differences are reflected in the structures, and how these structural differences are related to function. The aim of this review is to describe to what extent this structural motif for GDP/GTP binding is common to other known structures of this class of proteins. We first describe the common structural core of the G-proteins. Next, examples are based on information available on the Ras protein superfamily, the targeting protein ARF, elongation factors EF-Tu and EF-G, and the heterotrimeric G-proteins. Finally, we discuss the important structures of complexes between GTP binding proteins and their substrates that have appeared in the literature recently.