C2PA is a nuclear protein implicated in the heat shock response

C2PA is a protein of unknown function that is expressed in spermatocytes. PDZ-RGS3 is a signaling molecule whose PDZ domain binds Ephrin-B2 and mediates reverse signaling of this protein. C2PA and PDZ-RGS3 have identical PDZ domains. To explore the function of C2PA, we compared it with PDZ-RGS3 with respect to tissue distribution, subcellular localization, and biochemistry. C2PA is expressed only in testis, whereas PDZ-RGS3 is expressed in various tissues including brain, heart, lung, liver, spleen, kidney, small intestine, skeletal muscles, and testis. These proteins also differ in their subcellular distribution, in that PDZ-RGS3 is cytosolic while C2PA is exclusively nuclear. C2PA is distributed diffusely in the nucleus and forms a few foci at 37degreesC. However,when cells are exposed to 42degreesC, the number of C2PA foci is increased. These heat shock-induced foci colocalize with CREB-binding protein and heat shock factor-1. In contrast, the distribution of PDZ-RGS3 does not change during heat stress. When overexpressed, C2PA induces heat shock response element (HSE)-dependent gene transcription, whereas PDZ-RGS3 does not. These data suggest that the function of C2PA is distinct from that of PDZ-RGS3, and that C2PA may be involved in the heat shock response in testis.