Identification of cyclophilin A as a CD99-binding protein by yeast two-hybrid screening

CD99 is a 32kDa surface glycoprotein, which is involved in the migration of leukocytes and the transport of ganglioside GM1 and transmembrane proteins. To identify signaling mechanisms triggered by CD99 engagement, a LexA-based yeast two-hybrid system was utilized to identify proteins interacting with the cytoplasmic domains of CD99. In seven positive clones, we attempted to ascertain whether cyclophilin A (CypA) was involved in CD99-mediated signaling, since CypA had been implicated as a signaling regulator for kinases and phosphatases. The interaction between CD99 and CypA was confirmed by co-immunoprecipitation and confocal immunofluorescence studies. Interestingly, the amounts of CypA associated with CD99 increased upon CD99 engagement. We prepared an expression plasmid by inserting CypA cDNA into pEGFP, in order to visualize cellular CypA. In HeLa or HEK 293T cells transfected with the pEGFP-CypA plasmid, GFP-tagged CypA was diffusely present in the cytoplasm of untreated cells. However, CypA-GFP moved to the cell periphery and membrane blebbing, and became colocalized with CD99 upon CD99 engagement. These results suggest that CypA may be either a signaling mediator or a signaling regulator for CD99.