Interactions between the amino‐terminal domain of p56^lck and cytoplasmic domains of CD4 and CD8α in yeast

The interactions between CD4 or CD8 and p56^lck were tested using the twohybrid protein interaction system in yeast. Plasmid constructs were created which fuse the cytoplasmic domains of either CD4 or CD8α to the DNA‐binding protein LexA, and the unique amino‐terminal domain of p56^lck fused to a transcriptional activation domain. These constructs were transfected into yeast bearing lacZ and LEU2 reporter genes controlled by upstream LexA operator sequences. Yeast transfectants bearing either CD4 or CD8α hybrid proteins in combination with the amino terminal p56^lck hybrid protein exhibited increased β‐galactosidase activity and growth on leucine‐deficient medium, indicating interactions between these protein domains. Quantitation of reporter activation indicated that the interaction of p56^lck with CD8α is at least 18‐fold weaker than the interaction with CD4 in this assay. This reduced interactive capacity is apparently not due to competition by CD8α interacting with itself, since homotypic or heterotypic interactions between CD8α and/or CD4 could not be detected. Truncation and point mutants demonstrated that the interactions of p56^lck with CD4 or CD8α were dependent on the integrity of a pair of cysteines on each protein. The results indicate that these interactions do not require any additional proteins. Additionally, expression of the entire p56^lck molecule as a hybrid with LexA resulted in dramatic reduction in the growth of yeast. Though the two‐hybrid system is a powerful tool for examining protein interactions, this result indicates potential limitations in studying full‐length src family tyrosine kinases in yeast.