Sus1 is a central component of the yeast gene gating machinery, the process by 
which actively transcribing genes such as GAL1 become associated with nuclear 
pore complexes. Sus1 is a component of both the SAGA transcriptional 
co-activator complex and the TREX-2 complex that binds to nuclear pore 
complexes. TREX-2 contains two Sus1 chains that have an articulated helical 
hairpin fold, enabling them to wrap around an extended alpha-helix in Sac3, 
following a helical hydrophobic stripe. In SAGA, Sus1 binds to Sgf11 and has 
been proposed to provide a link between SAGA and TREX-2. We present here the 
crystal structure of the complex between Sus1 and the N-terminal region of Sgf11 
that forms an extended alpha-helix around which Sus1 wraps in a manner that 
shares some similarities with the Sus1-Sac3 interface in TREX-2. However, the 
Sus1-binding site on Sgf11 is somewhat shorter than on Sac3 and is based on a 
narrower hydrophobic stripe. Engineered mutants that disrupt the Sgf11-Sus1 
interaction in vitro confirm the importance of the hydrophobic helical stripe in 
molecular recognition. Helix alpha1 of the Sus1-articulated hairpin does not 
bind directly to Sgf11 and adopts a wide range of conformations within and 
between crystal forms, consistent with the presence of a flexible hinge and also 
with results from previous extensive mutagenesis studies (Klöckner, C., 
Schneider, M., Lutz, S., Jani, D., Kressler, D., Stewart, M., Hurt, E., and 
Köhler, A. (2009) J. Biol. Chem. 284, 12049-12056). A single Sus1 molecule 
cannot bind Sgf11 and Sac3 simultaneously and this, combined with the structure 
of the Sus1-Sgf11 complex, indicates that Sus1 forms separate subcomplexes 
within SAGA and TREX-2.