EVIDENCE THAT HUMAN-MILK ISOLATED CYCLOPHILIN-B CORRESPONDS TO A TRUNCATED FORM

Cyclophilin B (CyPB) is a member of the cyclophilin family (cyclospori n A-binding proteins) with specific N- and C-terminal extensions. In c ontrast to cyclophilin A, CyPB owns a signal sequence leading to its t ranslocation in the endoplasmic reticulum. CyPB was reported to be pre sent in human blood and milk, suggesting it is secreted. For this purp ose, CyPB was purified to homogeneity from human milk and compared to recombinant CyPB expressed in E. coli. Ion spray mass spectrometry rev ealed that CyPB secreted in human milk exhibits a lower molecular mass than the one expected. Identification of phenylalanine as the C-termi nus amino-acid residue of human milk CyPB indicates that the differenc e in molecular mass may be explained by the absence of the five C-term inal amino-acid residues AIAKE. These results suggest that in the sequ ence VEKPFAIAKE known to be responsible for retention of CyPB in the e ndoplasmic reticulum, the sequence AIAKE is more particularly necessar y. Our findings raise the possibility that the CyPB may be processed t o promote its release, As recombinant CyPB was shown to bind specifica lly to Jurkat cells, a lymphoblastic T-cell line, we then wanted to in vestigate the binding of human milk CyPB to these cells. Despite lacki ng the five C-terminal amino-acid residues, human milk CyPB is able to inhibit the binding of recombinant CyPB to Jurkat T cells.