When a preparation of plasma membranes was treated with a high salt (0.5M NaCl) solution,
it was observed that 40% of the total protein in the preparation was solubilised.

State what kind of membrane proteins are in this soluble extract and which
forces/interactions normally hold them in place.

Question

When a preparation of plasma membranes was treated with a high salt (0.5M NaCl) solution, 
it was observed that 40% of the total protein in the preparation was solubilised. 

 State what kind of membrane proteins are in this soluble extract and which 
forces/interactions normally hold them in place.

frostbite · Answer

When having a high salt concentration, we most be after proteins that primary use ionic bonding.
Proteins that does that in a membrane would be peripheral membrane proteins or integral membrane proteins... 
I doubt heavily that transmembrane proteins would be removed. Integral membrane proteins also have strong interactions, so I believe that only peripheral membrane proteins would be removed.
I would guess that the ionic bonds are being created from the proteins charged side groups and then interact with changed groups on the phospholipid or other proteins. Hydrogen bonding is also an option as polar side groups could react with the hydrophilic group on the phospholipid.

frostbite · Answer

An argument against integral membrane proteins would be hydrophobic interactions between hydrophobic side groups and the membrane.