Chimeric analysis of the small GTPase RacE in cytokinesis signaling in Dictyostelium discoideum.

RacE is a small GTPase required for cytokinesis in Dictyostelium discoideum. To investigate RacE's potential binding and signaling interfaces that allow its function in cytokinesis, 10 different chimeras were created between RacE and the closely related small GTPase, RacC. RacE/RacC chimeras, containing various combinations of four RacE regions, E I-IV: E-I (aa 1-67), E-II (aa 68-124), E-III (aa 125-184), and E-IV (aa 185-223), were tested for their ability to rescue the multinucleated, cytokinesis-defective phenotype of RacE null cells grown in suspension. Regions E-II and E-IV were essential but not sufficient for the rescue of RacE null cells. These two regions, in combination with either region E-1 or E-III, resulted in rescue. Results presented here suggest that region E-II contains a crucial, yet incomplete, binding site. Regions E-I or E-III separately provide additional, necessary elements for RacE's function. The extended E tail of RacE (E-IV) may act as a 'sensor' of the bound nucleotide state of RacE and facilitate GDP to GTP exchange (possibly through interactions with a GEF molecule), thereby resulting in activation of RacE. This study provides new evidence for small GTPases engaging several distinct protein interfaces to mediate signaling in various cellular processes.

The three-dimensional model of Dictyostelium discoideum racE based on the human rhoA-GDP crystal structure.

The three-dimensional structure of racE was modeled using several homologous small G proteins, and the best model obtained using the human rhoA as modeling template is reported. The three-dimensional fold of the racE model is remarkably similar to the cellular form of human ras p21 crystal structure. Its secondary structure consists of six alpha-helices, six beta-strands and three 3(10) helices. The model retains its secondary structure after a 300 K, 300 ps molecular dynamics (MD) simulation. Important domains of the protein include its effector loop (residues 34-46), the insertion domain (residues 121-136), and the polybasic motif (between 210 and 220) not modeled in the current structure. The effector loop is inherently flexible and the structure docked with GDP exhibits the effector loop moving significantly closer to the nucleotide binding pocket, forming a tighter complex with the bound GDP. The mobility of the effector loop is conferred by a single residue 'hinge' point at residue 34Asp, also allowing the Switch I region, immediately preceding the effector loop, to be equally mobile. In comparison, the Switch II region shows average mobility. The insertion domain is highly flexible, with the insertion taking the form of a helical domain, with several charged residues forming a complex charged interface over the entire insertion region. While the GDP moiety is loosely held in the active site, the metal cation is extensively co-ordinated. The critical residue 38Thr exhibits high mobility, and is seen interacting directly with the metal ion at a distance of 2.64 A, and indirectly via an intervening water molecule. 64Gln, a key residue involved in GTP hydrolysis in ras, is seen facing the beta-phosphate group and the metal ion. Certain residues (i.e. 51Asn, 38Thr and 65Glu) exhibit unique characteristics and these residues, together with 158Val, may play important roles in the maintenance of the protein's integrity and function. There is strong consensus of secondary structural elements between models generated using various templates, such as h-rac1, h-rhoA and h-cdc42 bound to RhoGDI, all sharing only 50-55% sequence identity with racE, which suggests that this model is in all probability an accurate prediction of the true tertiary structure of racE.