EFFECTS OF AMINO-TERMINUS TRUNCATION IN HUMAN CYTOCHROME P450IID6 ON ITS INSERTION INTO THE ENDOPLASMIC-RETICULUM MEMBRANE OF SACCHAROMYCES-CEREVISIAE

A truncated form of cytochrome P450IID6 deprived of 22 NH2-terminal am ino acids residues (P450IID6 Delta 1-22) was found in both the cytosol and the microsomal fraction of the yeast, Saccharomyces cerevisiae. A reduced CO difference spectrum of this form was characterized by the absence of absorption at 448 nm and weak absorption at 420 nm. Another peculiarity of P450IID6 Delta 1-22 expression was its reduced content in the yeast cells compared to that of P450IID6, with the intracellul ar levels of the corresponding mRNAs being the same. We suggest that t he deleted form of P450IID6, i.e. lacking 22 NH2-terminal amino acid r esidues, is not inserted properly in the endoplasmic reticulum membran e: it does not take up the proper conformation to enable normal heme b inding and is degraded in the yeast cells.