We perform systematic study of the effects of sequence-independent backbone interactions and sequence-dependent side-chain interactions
on protein folding using fragment assembly and physical energy function. Structures for ten proteins belonging to various structural classes 
are predicted only with Lennard-Jones interaction between backbone atoms. We find native-like structures for beta proteins, suggesting that 
for proteins in this class, the global tertiary structures can be determined mainly by sequence-independent backbone interactions.
On the other hand, for alpha proteins, non-local hydrophobic side-chain interaction is also required to obtain native-like structures.