Passenger protein determines translocation versus retention in the endoplasmic reticulum for aromatase expression.

Aromatase protein is overexpressed in the breasts of women affected with cancer. In the endoplasmic reticulum (ER), signal sequence and signal anchors (SAs) facilitate translocation and topology of proteins. To understand the function of type-I SAs (SA-Is), we evaluated translocation of aromatase, whose signal anchor follows a hydrophilic region. Aromatase SA-I mediates translocation of a short N-terminal hydrophillic domain to ER lumen and integrates the protein in the membrane, with the remainder of the protein residing in the cytosol. We showed that lack of a signal peptidase cleavage site is not responsible for the stop-transfer function of SA-I. However, SA-I could not block the translocation of a full-length microsomal secretory protein and was cleaved as part of the signal sequence. We propose that interaction between the translocon and the region after the signal anchor plays a critical role in directing the topology of the protein by SA-Is. The positive charges in the signal sequence helped it to override the function of signal anchor. Thus, when signal sequence follows SA-I immediately, the interaction with the translocon is perturbed and topology of the protein in ER is altered. If signal sequence is placed far enough from SA-I, then it does not affect membrane integration of SA-I. In summary, we conclude that it is not just the SA-I, but also the region following it, which together affect function of aromatase SA-I in ER.