What is unusual about the membrane spanning part of the hemolysin protein, with respect to the protein folding rules for soluble proteins?

The hemolysin protein refers to a membrane protein, which spans the phospholipid bilayer. In the hemolysin protein, the membrane-spanning part comprises beta sheets rolled into a barrel configuration rather than being made up of alpha-helices.  

When folded into a beta-barrel configuration, the outside of the barrel is hydrophobic and the interior is hydrophilic. A beta barrel configuration is essential for the generation of large, trans-membrane pores via which solute can diffuse hysterically, leading to hemolysis.