How many ways protein protein interactions are regulated? - (Mar/15/2014 )

I wonder how many ways protein-protein interactions are regulated. I know a lot of protein protein interactions are modified by phosphorylaton or other modification.

Is there other ways that mediate the protein protein interaction? Thanks!

Sure - presence of a co-factor, conformation, pH, degradation, environment (e.g. membrane proteins), are a few that spring to mind.  There are bound to be more.