Assembly of the Sm-class of U-rich small nuclear ribonucleoprotein particles (U 
snRNPs) is a process facilitated by the macromolecular survival of motor neuron 
(SMN) complex. This entity promotes the binding of a set of factors, termed 
LSm/Sm proteins, onto snRNA to form the core structure of these particles. Nine 
factors, including the SMN protein, the product of the spinal muscular atrophy 
(SMA) disease gene, Gemins 2-8 and unrip have been identified as the major 
components of the SMN complex. So far, however, only little is known about the 
architecture of this complex and the contribution of individual components to 
its function. Here, we present a comprehensive interaction map of all core 
components of the SMN complex based upon in vivo and in vitro methods. Our 
studies reveal a modular composition of the SMN complex with the three proteins 
SMN, Gemin8, and Gemin7 in its center. Onto this central building block the 
other components are bound via multiple interactions. Furthermore, by employing 
a novel assay, we were able to reconstitute the SMN complex from individual 
components and confirm the interaction map. Interestingly, SMN protein carrying 
an SMA-causing mutation was severely impaired in formation of the SMN complex. 
Finally, we show that the peripheral component Gemin5 contributes an essential 
activity to the SMN complex, most likely the transfer of Sm proteins onto the U 
snRNA. Collectively, the data presented here provide a basis for the detailed 
mechanistic and structural analysis of the assembly machinery of U snRNPs.