Therefore, specificity of outer segment targeting in this approach is indicated by the lack of the construct’s “spillage” outside the outer segment. Previous study of peripherin��s C-terminus found that deletions affecting a twenty amino acid stretch resulted in loss of exclusive targeting to outer segments. However, it was not examined if this sequence alone was sufficient to target a reporter to the outer segment. Therefore, we first investigated whether peripherin��s reported targeting sequence is able to specifically direct the YFP-xRhoCTD5 reporter to the outer segment and found that it was. We next asked whether all twenty amino acids are required for targeting, particularly because the first ten amino acids in this sequence overlap with peripherin��s previously identified fusogenic region located within residues 311�C 325. In order to test whether these two functional regions overlap or are separate from one another, we generated constructs in which the reporter was fused to either the first ten amino acids or the last ten amino acids of the targeting region. Only the latter AbMole Acetylcorynoline construct targeted exclusively to the outer segment demonstrating that the peripherin targeting signal is wholly contained within a ten amino acid residue stretch and that it is distinct from peripherin’s fusogenic region. Interestingly, the expression of the construct containing a portion of peripherin’s fusogenic region caused outer segments to appear distorted, which is particularly well-seen in rods displaying strong fluorescent signal. Our construct encompasses a part of an amino acid sequence promoting membrane fusion in vitro, including two of the three residues most critical for this function, Glu321 and Lys324. This may explain why expression of this construct disrupts outer segment membranes. However, expression of a longer C-terminal construct did not result in irregular outer segment morphology. One potential explanation for this difference is that membrane fusion by peripherin is likely to be a highly regulated process that occurs only during disc morphogenesis. Accordingly, this process would need to be prevented during the rest of the lifetime of peripherin. It could be further speculated that inhibitors of peripherin’s fusogenic activity can interact with longer, but not shorter, transgenic constructs thereby preventing disruption of outer segment membranes. An alternative approach to characterize the sufficiency of peripherin��s targeting sequence for outer segment protein delivery is to test whether it could redirect intracellular trafficking of a protein reporter otherwise targeted to another subcellular compartment. For this purpose, we selected the Htr1a serotonin receptor because it was previously shown to be excluded from cilia in other cell types. On the other hand, when fused to the rhodopsin C-terminus this receptor was shown to be delivered to rod outer segments of transgenic Xenopus. We first demonstrated that the YFP-fused Htr1a construct was completely excluded from rod outer segments of transgenic Xenopus, in agreement with observations in other ciliated cells. This construct distributed throughout the plasma membrane of the inner segment and synaptic terminal and was prominently present in the calycal processes, which are microvillar extensions of the inner segment plasma membrane surrounding the outer segment.