In situ chemical cross-linking on living cells reveals CD9P-1 cis-oligomer at cell surface.

Tetraspanins are integral membrane proteins involved in a variety of physiological and pathological processes. They associate with each other in multimolecular complexes containing numerous membrane proteins. As a first step towards the study of the supramolecular organization of tetraspanin complexes, we have implemented a proteomic approach based on in situ protein cross-linking on living cells followed by affinity purification of tetraspanin complexes. This allowed observing the presence of high molecular weight protein complexes that were characterized as containing CD9P-1/CD315 using LC-MS/MS. Western blot analyses and the use of different tags demonstrated the presence of CD9P-1 oligomer in cis-association at cell surface. A significant amount of CD9P-1 oligomer was observed on various cell types. We have shown that CD9P-1 self-associates independently from its association with tetraspanins. However, the expression level of CD9 or CD81 that associate directly and specifically with CD9P-1, positively modulates the cross-linking efficiency of CD9P-1. Thus, tetraspanins can play a role on CD9P-1 oligomerization status.