The target of rapamycin (Tor) is a Ser/Thr protein kinase that regulates a range 
of anabolic and catabolic processes. Tor is present in two complexes, TORC1 and 
TORC2, in which the Tor-Lst8 heterodimer forms a common sub-complex. We have 
determined the cryo-electron microscopy (EM) structure of Tor bound to Lst8. Two 
Tor-Lst8 heterodimers assemble further into a dyad-symmetry dimer mediated by 
Tor-Tor interactions. The first 1,300 residues of Tor form a HEAT 
repeat-containing α-solenoid with four distinct segments: a highly curved 
800-residue N-terminal 'spiral', followed by a 400-residue low-curvature 
'bridge' and an extended 'railing' running along the bridge leading to the 'cap' 
that links to FAT region. This complex topology was verified by domain 
insertions and offers a new interpretation of the mTORC1 structure. The spiral 
of one TOR interacts with the bridge of another, which together form a joint 
platform for the Regulatory Associated Protein of TOR (RAPTOR) regulatory 
subunit.