3.2. Tags and other fusion partners
In addition to the open reading frame (ORF) of the gene of interest, a typical expression plasmid will usually incorporate a number of other sequences in its expression cassette. The S. cerevisiae α-mating factor signal sequence is a common addition to commercial expression plasmids ( Table 1) because it is believed to correctly-target recombinant membrane proteins to the yeast membrane. For example, its presence had a positive impact on the yield of the mouse 5-HT5A serotonin receptor [27] but dramatically reduced expression of the histamine H1 receptor [28]. Alternative signal sequences have been used (albeit much less frequently) such as the STE2 leader sequence of the fungal GPCR, Ste2p