Fetal antigen 2 (FA2): the aminopropeptide of the alpha 1 chain of human type I procollagen. A study on skin, tumour biology and bone metabolism.

Fetal antigen 2 (FA2) was purified from second trimester human amniotic fluid (AF) using immunospecific chromatography, gel filtration and reversed phase chromatography. Amino acid composition and sequence analyses of purified FA2 revealed identity to the amino acid sequence of the aminopropeptide of the alpha 1 chain of human type I procollagen. Gel filtration of AF as well as purified FA2 demonstrated two molecular forms of FA2, which during gel filtration were elucidated corresponding to MW of approximately 100 kDa and 30 kDa. The molecular forms were referred to as the high molecular component (FA2-HM) and low molecular component (FA2-LM), respectively. SDS-PAGE analysis (reducing and nonreducing conditions) showed a MW of 27 kDa for both FA2-HM and FA2-LM. Mass spectrometry analysis however revealed a MW of 14.343 +/- 3Da. Proteins with collagenous domains often reveal aberrant behaviour both in gel filtration and SDS-PAGE and this phenomenon may account for the apparent higher MW of FA2 observed using these techniques compared to mass spectrometry and amino acid composition/sequence analyses. A polyclonal, monospecific rabbit antibody raised against FA2 was used in immunohistochemical localization studies. Due to the obvious association of FA2 to the BM structure in adult and fetal skin, it was initially referred to as a BM associated component. However, FA2 was immunologically distinct from the well established BM components and moreover, the diffuse distribution of FA2 along the BM differed from linear localization of BM components (e.g. collagen type IV and laminin). The identification of FA2 as the aminopropeptide of the alpha 1 chain of human procollagen type I has the implication that in the immunohistochemical investigations the anti-FA2 may not distinguish between the free propeptide and procollagen type I. In the papillary dermal region of human adult skin type I collagen are arranged in loose fibrils to which the aminoterminal propeptide is still attached. Anti-FA2 may recognize epitopes in the type I procollagen present in this region, giving rise to the diffuse FA2 distribution along the BM. However, whether FA2 also appear as a free component in this region is unknown. In fetal skin where collagen is vividly layed down in the mesenchymal compartment staining with anti-FA2 revealed a diffuse mesenchymal distribution. Demonstration of FA2 within the cytoplasm of proliferating fibroblasts and diffusely in granulation tissue during wound healing suggest that FA2 was involved in the reparative processes.(ABSTRACT TRUNCATED AT 400 WORDS)