The presence of two basic amino acids strategically located within a single 
spanning transmembrane region has previously been shown to act as a signal for 
the endoplasmic reticulum associated degradation (ERAD) of several polypeptides. 
In contrast, the functionality of this degron motif within the context of a 
polytopic membrane protein has not been established. Using opsin as a model 
system, we have investigated the consequences of inserting the degron motif in 
the first of its seven transmembrane (TM) spans. Whilst these basic residue 
reduce the binding of the targeting factor, signal recognition particle, to the 
first TM span, this has no effect on membrane integration in vitro or in vivo. 
This most likely reflects the presence of multiple TM spans that can act as 
targeting signals within in the nascent opsin chain. We find that the degron 
motif leads to the efficient retention of mutant opsin chains at the endoplasmic 
reticulum. The mutant opsin polypeptides are degraded via a proteasomal pathway 
that involves the actions of the E3 ubiquitin ligase HRD1. In contrast, 
wild-type opsin remains stable for a prolonged period even when artificially 
accumulated at the endoplasmic reticulum. We conclude that a single dibasic 
degron motif is sufficient to initiate both the ER retention and subsequent 
degradation of ospin via an ERAD pathway.