Cloning of three variants of type XVIII collagen and their expression patterns during Xenopus laevis development

Xenopus laevis type XVIII collagen occurs in three variants, 22 + 1285 amino acid residues (signal peptide + mature protein), 23 + 1581 residues and 23 + 1886 residues in length, differing in their N-terminal non-collagenous domains. The region showing highest homology to mammalian counterparts is the C-terminal endostatin domain. All three variants are expressed, at different levels, during early and late stages of development, as demonstrated by reverse transcription-polymerase chain reaction. Whole-mount in situ hybridization shows that the short variant is expressed at high levels in the developing eye, the central nervous system, the otic vesicle, the head mesenchyme, the branchial arches and the pronephros, and at the boundaries between somites. The middle variant is expressed in the head mesenchyme, the branchial arches, the peripheral nervous system, the pronephros and the pronephric duct, and at the somite boundaries. The longest variant is weakly expressed in the head mesenchyme and branchial arches.