The type I keratin 19 possesses distinct and context-dependent assembly properties

Keratins (K), the cytoplasmic intermediate filament (IF) proteins of epithe lial cells, are encoded by a multigene family and expressed in a tissue- an d differentiation-specific manner. In human skin, keratinocytes of the basa l layer of epidermis and the outer root sheath of hair follicles express K5 and K14 as their main keratins. A small subpopulation of basal cells exhib iting stem-cell Like characteristics express, in addition, K19. At 40 kDa, this keratin is the smallest LF protein due to an exceptionally short carbo xyl-terminal domain. We examined the assembly properties of K19 and contras ted them to K14 in vitro and in vivo. Relative to K5-K14, we find that K5-K 19 form less stable tetramers that polymerize into shorter and narrower Ifs in vitro. When transiently coexpressed in cultured baby hamster kidney cel ls, the K5 and K19 combination fails to form a filamentous array, whereas t he K5-K14 and K8-K19 ones readily do so. Transient expression of K19 in the epithelial cell lines T51B-Ni and A431 results in its integration into the endogenous keratin network with minimal if any perturbation. Collectively, these results indicate that K19 possesses assembly properties that are dis tinct from those of K14 and suggest that it may impart unique properties to the basal cells expressing it in skin epithelia.