The structure of the Guanine Nucleotide Exchange Factor Rlf in complex with the small G-protein Ral identifies conformational intermediates of the exchange reaction and the basis for the selectivity.

CDC25 homology domain (CDC25-HD) containing Guanine Nucleotide Exchange Factors (GEFs) initiate signalling by small G-proteins of the Ras-family. Each GEF acts on a small subset of the G-proteins only, thus providing signalling selectivity. Rlf is a GEF with selectivity for the G-proteins RalA and RalB.

The guanine nucleotide exchange factor Rlf interacts with SH3 domain-containing proteins via a binding site with a preselected conformation.

Rlf is a guanine nucleotide exchange factor for the small G-proteins RalA and RalB and couples Ras- to Ral-signalling. Here the crystal structure of the catalytic module of Rlf consisting of a REM- and a CDC25-homology domain is determined. The structure is distinguished by an extended three stranded β-sheet called the flagpole.

Selectivity of CDC25 homology domain-containing guanine nucleotide exchange factors.

The Ras family of small G-proteins plays an essential role in the regulation of a variety of signal transduction processes, ranging from cell cycle control to the regulation of exocytosis. Signalling by the Ras G-proteins is initiated by the CDC25 homology domain (CDC25-HD) containing guanine nucleotide exchange factors (GEFs); each GEF, with its specific selectivity profile towards G-proteins, commonly acts on only a small subset of the Ras family members. Thus, GEFs play a pivotal part in establishing the activation of the downstream signalling routes.