Function:

 

Part of the RAS superfamily, the main function of cdc42 is to regulate the formation of actin filaments, therefore, cell growth and differentiation. cdc42 also regulates gene expression and is involved in transforming normal cells into malignant ones. This G-protein is essentially a molecular switch, and so it is either in its active GTP bound or inactive GDP bound conformational state.  Guanine nucleotide exchange factors are essential for activating cdc42, further to this, GTPase activating proteins inactivate cdc42 through catalysing the hydrolysis of GTP to GDP. 

 

 

Regulation of CDC42: 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Terminals:

 

Sequences at their COOH termini that undergo posttranslational modifications with lipids, such as farnesyl, geranylgeranyl, palmitoyl, and methyl moieties, and proteolysis. Two thirds of the carboxy terminal (amino acids 74-204) make up a domain called the geranylgeranyl-binding domain. This domain is responsible for extraction of CDC42 from the cell membrane by GDI (Guanosine Dissociation Inhibitor). 

 

Switch regions: 

 

The switch regions exhibit much higher inherent mobility compared to the rest of the G domain. Conformational changes are triggered in the switch regions when two hydrogens bond to the gamma phosphate from switch I and II. This involves both threonine and glycine residues and they are released after GTP has been hydrolysed to GDP. GDI contains a "regulatory arm" that causes inhibition of GDP dissociation and GTP hydrolysis via interactions with the Switch regions.

 

Switch I:

 

Swtich I is the loop2-beta2 region of cdc42 and is a relatively flexible and dynamic domain, which is required for function. Experimental evidence demonstrates that minor modifications in this region alter its ability to interact with effectors along with the current hypothesis that it also affects its interactions with regulators. Modifications of switch I's conformational flexibility has been shown to slow proteolytic cleavage relative to wild type cdc42 switch I regions. Thr(35) seems to be critical for function, as it is completely invariant in Ras-related proteins. The methyl group in the Thr(35) side chain is responsible for cdc42's dynamic behaviour and the side chain hydroxly is involved with the coordination of metal ions via the NH in its main chain in contracting the gamma phosphate. The dynamic and flexible properties of the ras superfamily has resulted in the conservation of the only invariant residue, Thr(35).

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Switch II:

 

Switch II is the beta3-alpha2 region of cdc42. This switch is made up of two helices, the first of which is amino acids 62-64 and the second one consists of residues 68-70. Unlike many other GTPases, Switch II is mostly nucleotide independent (the conformation doesn't change that much when GDP is replaced by GTP). This nucleotide independence is thought to be due to the fact that GDI can bind to cdc42 whether it is bound to GTP or GDP. Contact between Switch II and GDI is the second most important interaction (the geranylgeranyl-binding pocket has the first most important interations). Hydrogen bonds and non-polar bonds stabilise this interaction. These contacts form a well defined conformation and are important in inhibiting the hydrolysis of GTP, which is caused by GDI binding.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

References

 

1. Chandrashekar. R, Salem. O, Krizova. H, McFeeters. R, Adams. P. D. 2011. A switch I mutant of Cdc42 exhibits less conformational freedom. Biochemistry. 50(28):6196-207

 

2. Hoffman, G. R., Nassar, N. Cerione, R. A. 2000. Structure of the Rho Family GTP- Binding Protein Cdc42 in Complex with the Multifunctional Regulator RhoGDI. Cell, 100, 345-356.

 

3. Johnson, D. I. 1999. Cdc42: An Essential Rho- Type GTPase Controlling Eukaryotic Cell Polarity. Microbiology and Molecular Biology Reviews, 63, 54.