SMALL PROLINE-RICH PROTEINS ARE CROSS-BRIDGING PROTEINS IN THE CORNIFIED CELL ENVELOPES OF STRATIFIED SQUAMOUS EPITHELIA

The cornified cell envelope (CE) is a specialized structure which cont ributes barrier function to stratified squamous epithelial cells. It i s composed of an amalgam of several structural proteins that are rende red insoluble by isopeptide bond crosslinking by transglutaminases, On e set of the structural proteins present in CEs of most such epithelia are the small proline rich (SPR) proteins, which are a family of abou t 12 related structural proteins. We have recovered a large number of peptides containing isopeptide crosslinks, including 236 involving SPR proteins, following proteolysis of CEs isolated from foreskin epiderm al tissue and cultured epidermal keratinocytes. Analysis of this datab ase has provided novel information on their function. First, we found that SPRs became crosslinked to many other structural proteins within the CE. Second, multiple glutamine and lysine residues located only on the amino- and carboxy-termini of the SPR proteins were involved in c rosslinking, so that the two ends are functionally equivalent. Third, the SPRs functioned as cross-bridging proteins, by directly adjoining other CE structural proteins. In the specialized case of the epidermal CE, the SPRs cross-bridged between loricrin. In cultured keratinocyte s which make little loricrin and serve as a model for internal stratif ied squamous epithelia, the SPRs formed extensive cross-bridges among themselves. Thus SPRs are ubiquitous cross-bridging proteins whose dif ferential expression patterns apparently reflect specific barrier requ irements of different epithelia.