Signal Sequence Trap: Expression Cloning Method for Secreted Proteins and Type 1 Membrane Proteins

Intercellular signaling and cell adhesion are among the most critical mechanisms for development and maintenance of multicellular organisms. Although a large number of molecules involved in signaling or adhesion have been cloned, there still remain many unknown molecules that are important in these functions. Most of the molecules involved in signaling or adhesion are secreted or membrane-anchored proteins. Many of these proteins contain a signal sequence or leader peptide in the N-terminal of their premature form (1 ), The traditional strategy for cloning these genes requires a functional assay to detect the specific function of each molecule. To establish a general cDNA cloning method for growth factors, hormones, neuropeptides, their receptors, and adhesion molecules, we have developed a new cloning strategy for selecting cDNA fragments encoding N-terminal signal sequences (2 ). This method, called Signal Sequence Trap, turns out to be an efficient method to isolate 5′-cDNA fragments from secreted or transmembrane proteins. We have already obtained a number of cDNA clones of putative growth factors, receptors, or adhesion molecules (2 –5 ). Signal Sequence Trap can clone not only secreted proteins, GPI-anchored proteins, and plasma membrane proteins, but also proteins located in endoplasmic reticulum (ER), Golgi apparatus (GA), and lysosome.