Ras homologue enriched in brain (Rheb) represents a unique group of small 
GTPases and shares moderate sequence identity with the Ras/Rap subfamily. It 
acts downstream of nutrient signalling as the direct target of the tuberous 
sclerosis complex (TSC) and upstream of mTOR/S6K1/4EBP in the insulin-signalling 
pathway. The GTPase domain of human Rheb (hRheb) has been recombinantly 
expressed in Escherichia coli, purified and cocrystallized in complexes with 
GDP, GTP and GppNHp using the hanging-drop vapour-diffusion method. Crystals of 
the hRheb-GDP complex belong to space group P2(1)2(1)2(1), with unit-cell 
parameters a = 44.5, b = 52.3, c = 70.6 A. The hRheb-GppNHp complex crystallized 
in two crystal forms: one has the same space group and unit-cell parameters as 
the hRheb-GDP complex and the other belongs to space group C222(1), with 
unit-cell parameters a = 102.9, b = 99.2, c = 48.0 A. The hRheb-GTP complex also 
crystallized in two crystal forms: one belongs to space group C222(1), with 
unit-cell parameters a = 102.4, b = 98.3, c = 47.9 A, and the other belongs to 
space group P2(1), with unit-cell parameters a = 77.3, b = 47.9, c = 71.9 A, 
beta = 89.0 degrees. All these crystals diffract X-rays to better than 2.8 A 
resolution and at least one diffraction data set has been collected for each 
crystal form using an in-house R-AXIS IV++ diffractometer. Structural studies of 
hRheb in complexes with various substrates may provide insights into the 
recognition and specificity of substrate and the catalytic mechanism of 
mammalian Rhebs and shed light on the biological functions of Rhebs in the mTOR 
signalling pathway.