The cheapest method to crosslink interacting proteins for Co-IP is to use
Formaldehyde as crosslinker. You need to find the lowest concentration of
formaldehyde in an initial test. I have been using 0.1% formaldehyde in PBS
to crosslink proteins on freshly harbested cells, room temperature for a few
minutes, and quench with 1.25M Glycine. Then lyse cells, sonicate and IP as
usual. I found this paper very useful: Optimization of Formaldehyde Cross-
Linking for Protein
Interaction Analysis of Non-Tagged Integrin β1