Abstract

Using two drugs, tunicamycin and brefeldin A, which affect protein processing, we investigated the intracellular processing mechanism of secreted aspartyl proteinase 1 (SAP1) of Candida albicans. Three intracellular forms of SAP1 were detected by immunoblotting using monoclonal antibody (MAb) CAP1. Their molecular weights were approximately 40, 41 and 45 kDa, respectively. The 41 kDa protein is a glycoprotein and may be the same as the extracellular form judging by its molecular mass. The 40 kDa protein was the unglycosylated form and its molecular mass coincided with deglycosylated SAP1 and the 45 kDa protein was also the unglycosylated form. Neither the 40 and 45 kDa proteins were detected in the culture supernatant of C. albicans. These suggested that the 40 and 45 kDa proteins might be intracellular precursor forms of SAP1. These results show that SAP1 is translated as a 45 kDa precusor form in the endoplasmic reticulum and the 45 kDa precursor form undergoes proteolytic cleavage after translocation into the Golgi apparatus, generating the 40 kDa precursor form. This 40 kDa precursor is converted into a 41 kDa mature form through glycosylation in the Golgi apparatus. The mature form of the 41 kDa protein is sorted into secretory vesicles and finally released into the extracellular space through membrane fusion. When the glycan region of SAP1 was digested with N-glycosidase F, both stability and activity of the enzyme decreased. These results indicate that the glycan attached to the enzyme may, at least in part, be related to enzyme stability and activity.

• Keywords: Candida albicans; aspartyl proteinase; intracellular modification; glycan