Isolation of Photoreceptor Cell-Specific MEKA cDNA by Differential Hybridization

All cells have a large number of proteins; for instance, a single hepatocyte has about lo⁴ different proteins, and a number of them are involved in tissue-specific functions. In neuronal cells, there are additional 10⁴ protein species, which are predicted, from kinetic of DNA-RNA hybridization studies, to be involved in specialized neural functions and in intricate systems of communication (1 ,2 ). At present, it is possible to separate and analyze numerous proteins using two-dimensional gel electrophoresis; however, only around 10³ protein species in a given tissue can be detected by this method even when combined with the silver-staining technique. In contrast, if a cDNA library from mRNAs is screened, using molecular biological techniques, about 10⁵ –10⁶ independent clones, each of which is expected to encode different proteins, can be easily isolated and screened. Recent advances in the techniques of genetic engineering encourage the neuroscientist to search for molecules underlying neuronal functions.