Abstract

Lysyl hydroxylase (E.C. 1.14.11.4) catalyzes the formation of hydroxylysine in collagens and other proteins with collagenous domains. The resulting hydroxylysine residues participate in the formation of collagen crosslinks, and serve as attachment sites for carbohydrate units. They have been regarded as non-essential, since the absence of lysyl hydroxylase 1 activity is not lethal, although it leads to the kyphoscoliotic type of Ehlers-Danlos syndrome, and since recombinant collagens I and III lacking any hydroxylysine form native-type fibrils in vitro.

A novel human lysyl hydroxylase isoenzyme, lysyl hydroxylase 3, was identified, cloned and characterized here. The novel isoenzyme was expressed as a recombinant protein in insect cells, and the protein was shown to catalyze hydroxylation of lysine residues in vitro. No differences were found in the catalytic properties between the recombinant lysyl hydroxylases 3 and 1.

The human lysyl hydroxylase 3 gene was shown to be 11.6 kb in size and to contain 19 exons. The introns contain 15 full-length or partial Alu retroposons, which are known to be involved in most human gene rearrangements that occur by homologous recombination.

The three recombinant human lysyl hydroxylase isoenzymes were isolated here for the first time as homogenous proteins. Limited proteolysis data suggested that the lysyl hydroxylase polypeptides might consist of at least three distinct domains, A-C. The N-terminal domain A was found to play no role in lysyl hydroxylase activity as a recombinant B-C polypeptide was a fully active hydroxylase. This work also confirmed that lysyl hydroxylase 3 has collagen glucosyltransferase activity as well as trace amounts of collagen galactosyltransferase activity. However, the levels of these activities were so low that their biological significance remains to be determined.

In the last part of this work, lysyl hydroxylase 3 knock-out mice were produced and analyzed. The homozygous null embryos were found to die at a very early stage of development due to lack of type IV collagen in the basement membranes. The data demonstrated that hydroxylysine formed by lysyl hydroxylase 3 is essential for early mouse development and that lysyl hydroxylase 1 or 2 cannot compensate for the lack of its function.