Proteolytic processing of the laminin α3 G domain mediates assembly of flemidesmosomes but has no role on keratinocyte migration

Laminin-5 (Lm5), the major adhesion ligand of basal epithelial cells, undergoes complex extracellular proteolytic processing that influences cell adhesion and migration. In tumor cell lines, the proteolytic truncation of the C-terminal G domain of the Lm alpha 3 chain induces assembly of hemidesmosomes and downregulates cell migration. To define the biological functions of the alpha 3 G domain processing in physiological conditions, we have expressed a series of mutant alpha 3 complementary DNA in human primary alpha 3-null keratinocytes immortalized by human papillomavirus E6E7 (HK alpha 3 cells). Using monolayer and organotypic cell cultures we show that: (1) the hinge region between subdomains G3 and G4 carries the proteolytic cleavage sites; (2) nucleation of the hemidesmosomal proteins is independent of the proteolytic maturation of the alpha 3 G domain, whereas formation of mature hemidesmosomes relies on proteolytic cleavage of alpha 3; and (3) the proteolytic processing plays no role in cell migration, which suggests that nucleation of hemidesmosomal structures in culture does not reflect the migratory potential of the epithelial cells. Our results also demonstrate that HK alpha 3 cells are a unique model system, which will be useful to dissect the functions and molecular interactions of Lm5.