TRANSLOCATIONAL PAUSING IS A COMMON STEP IN THE BIOGENESIS OF UNCONVENTIONAL INTEGRAL MEMBRANE AND SECRETORY PROTEINS

Signal, stop transfer, and signal-anchor sequences direct a nascent po lypeptide to a single topology with respect to the membrane of the end oplasmic reticulum. However, other types of sequences direct nascent p roteins, either transiently or permanently, to more than one topologic form. Pause transfer sequences direct nascent apolipoprotein B to pau se during its translocation, resulting in nonintegrated, transmembrane intermediates that become fully translocated over time. The stop tran sfer effector sequence (STE) directs the nascent prion protein either to integrate at the hydrophobic domain which immediately follows (TM1) or to become fully translocated, in a manner dependent on cytosolic f actors. Although the action of pause transfer sequences has been disse cted into stop and restart steps, the mechanism of STE action is unkno wn. Using chimeric proteins expressed in vitro, we show that STE, inde - pendent of TM1, acts as a pause transfer sequence. We also demonstra te that translocational pausing at STE is a common step preceding eith er complete translocation or integration into the membrane of a chimer ic protein containing STE and TM1. These findings have implications fo r the role of pausing in the biogenesis of both secretory and membrane proteins.