Outside of missing post translational modifications, can you give a concrete example of a protein that is known to fold differently in different species, not counting, say, stuff getting sent to the garbage bin of inclusion bodies due to the stress of overexpression? My understanding (7 years of grad school researching protein folding in the ER) is that outside of some rare corner and disease state cases, folding is pretty much binary event, and if it weren't for most cases the low delta g difference between isoforms would be just as easily overcome over the course of environmental changes in a single individual as "between different species" namely having a deterministic outcome is important for through-time robustness.