The mammalian target of rapamycin (mTOR) is part of two distinct complexes, 
mTORC1, containing raptor and mLST8, and mTORC2, containing rictor, mLST8 and 
sin1. Although great endeavors have already been made to elucidate the function 
and regulation of mTOR, the cytoplasmic nuclear distribution of the mTOR 
complexes is unknown. Upon establishment of the proper experimental conditions, 
we found mTOR, mLST8, rictor and sin1 to be less abundant in the nucleus than in 
the cytoplasm of non-transformed, non-immortalized, diploid human primary 
fibroblasts. Although raptor is also high abundant in the nucleus, the 
mTOR/raptor complex is predominantly cytoplasmic, whereas the mTOR/rictor 
complex is abundant in both compartments. Rapamycin negatively regulates the 
formation of both mTOR complexes, but the molecular mechanism of its effects on 
mTORC2 remained elusive. We describe that in primary cells short-term treatment 
with rapamycin triggers dephosphorylation of rictor and sin1 exclusively in the 
cytoplasm, but does not affect mTORC2 assembly. Prolonged drug treatment leads 
to complete dephosphorylation and cytoplasmic translocation of nuclear rictor 
and sin1 accompanied by inhibition of mTORC2 assembly. The distinct cytoplasmic 
and nuclear upstream and downstream effectors of mTOR are involved in many 
cancers and human genetic diseases, such as tuberous sclerosis, Peutz-Jeghers 
syndrome, von Hippel-Lindau disease, neurofibromatosis type 1, polycystic kidney 
disease, Alzheimer's disease, cardiac hypertrophy, obesity and diabetes. 
Accordingly, analogs of rapamycin are currently tested in many different 
clinical trials. Our data allow new insights into the molecular consequences of 
mTOR dysregulation under pathophysiological conditions and should help to 
optimize rapamycin treatment of human diseases.