short loop between domains 1 and 2 residues 84 to 88 and within the long loop between domains 2 and 3 residues 217 234 of lucy see fig1 b which depicts these split points schematically antiparallel leucine zippers were used as an idealized model for interacting protein partners ghosh et al 2000 because these leucine zippers interact in an antiparallel fashion one fragment of lucy was fused at the amino terminus of a leucine zipper split points spanz1 5 and spbnz1 5 while the complementary fragment was fused to the carboxy terminus of the partner leucine zipper spacz1 5 and spbcz1 5 this is depicted schematically in fig1 c the letters a and b and numbers 1 through 5 indicate the location of the split see table 3 matching split point numbers indicates that the cz split point immediately follows the residue at the carboxy end of the nz split point fusion pairs containing complementary lucy fragments e g spanz1 spacz1 spbnz4 spbcz4 etc were tested for protein expression and for fluorescence complementation fluorescence produced by coexpressed fragment pairs fused to leucine zippers was compared to fluorescence of fully intact lucy fused to either the n or c terminus of one of the zippers nz and cz fluorescence was determined visually in whole cell pellets and quantitatively by a fluorometer fragmenting lucy at any of the five residues between domains 1 and 2 did not result in fluorescence when fused to leucine pairs and coexpressed fig1 a a contributing factor may have been the low expression of the cz half of each of the split points in comparison to the complementary.