Figure 5.

Model for the organization of a-factor biogenesis components. The a-factor precursor is modified by several components. Isoprenylation is carried out by the cytosolic Ram1p/Ram2p complex. The Rce1p and Ste24p proteases and the Ste14p methyltransferase are localized to the ER membrane, presumably with cytosolically disposed active sites. Axl1p is localized to a different, as yet unidentified, compartment. The Ste6p transporter is localized to the plasma membrane. Routing through these processing stations is required for the maturation of the a-factor precursor and for the export of mature a-factor.