Testins are structurally related to the mouse cysteine proteinase precursor but devoid of any protease/anti-protease activity.

Testin I (M(r) 35,000) and testin II (M(r) 37,000) (testins) are two structurally and immunologically related testicular proteins that are actively synthesized and secreted by Sertoli cells. Treatment of adult rats with busulfan to destroy germ cells selectively in the testis could lead to a significant increase in the testicular testins level; as the germ cells reappeared in the testis, the testicular testins level declined and returned to the normal level. These observations indicated that the testicular content of testins in the rat is inversely correlated to the number of germ cells. When the partial N-terminal amino acid sequences for testin I and testin II were compared with the existing protein data base at Protein Identification Resource, it was noted that they displayed remarkable identity with CTLA-2 alpha and CTLA-2 beta, two novel molecules expressed in mouse activated T lymphocytes and mast cells, and the mouse cysteine proteinase proregion. When purified testins were assayed for the proteolytic and anti-protease activity using [14C]-casein, it was noted that it possessed neither proteolytic nor anti-protease activity, suggesting that it is not functioning as a protease and/or protease inhibitor in the testis.