A conserved region containing three to five proline residues is present just behind the signal-anchor sequence in the amino terminal portion of most microsomal cytochrome P-450s. We have shown that the proline residues are crucial for correct folding in Schizosaccharomyces pombe cells by using mutants of P-450(M1) in which one to three of the proline residues were changed to alanine. To examine the effects of the mutations on the intracellular localization of P-450s, they were expressed in COS-7 cells. They were found to be localized only in the perinuclear loci as patched structures like the Golgi apparatus, while the wild-type P-450(M1) is localized in the reticular structures which are typical for the ER membrane. However, treatment of the cells with Brefeldin A had no effect on the patched structures. Upon co-expression with another ER membrane protein, CD4D, which possesses a double lysine motif, the expressed CD4D was localized not only in the patched structures as the mutated P-450(M1)s, but also in the reticular structures of ER. When the cells were homogenized and then fractionated, the mutated P-450(M1) was recovered mainly in the low-speed precipitate and in the fractions of much higher density than the normal ER membrane. On electron microscopic observation, unusual membranous bodies were observed near the nucleus only when the mutated P-450(M1) was expressed.(ABSTRACT TRUNCATED AT 250 WORDS)