Protein S is an anticoagulant plasma protein, functioning as a cofactor to activated protein C in the regulation of blood coagulation. In addition, protein S forms a complex with the complement regulatory protein, C4b-binding protein. Protein S is unique among the vitamin K-dependent proteins in being structurally similar to androgen binding proteins. Protein S immunoreactivity was demonstrated in Leydig cells of human testis. In Northern blotting experiments, the presence of protein S mRNA in human testis tissue could be shown. In situ hybridization experiments localized protein S mRNA to the Leydig cells, demonstrating transcription of the protein S gene in these cells. Five protein S clones were isolated from a human testis cDNA library, partially sequenced and characterized by restriction enzyme mapping. Three unique clones contained information for the entire coding sequence and approximately two-thirds of the 5′ and 3′ non-coding sequences. The results indicate the nucleotide sequences of testis and liver protein S mRNA to be identical. No binding of androgens to protein S could be demonstrated. In conclusion, we demonstrate the presence of protein S immunoreactivity as well as protein S mRNA in the Leydig cells of human testis. These results suggest local synthesis of protein S in Leydig cells of human testis which may be functionally important for local anticoagulation.

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