Cross-linked envelope protein of keratinocytes. It is a keratinocyte protein that first appears in the cell cytosol, but ultimately becomes cross-linked to membrane proteins by transglutaminase. All that results in the formation of an insoluble envelope beneath the plasma membrane. Can function as both amine donor and acceptor in transglutaminase-mediated cross-linkage. This is the function of SPRR1B (small proline rich protein 1B, ENSG00000169469).