Assembly of stable human type I and III collagen molecules from hydroxylated recombinant chains in the yeast Pichia pastoris -: Effect of an engineered C-terminal oligomerization domain foldon

The C-propeptides of the proalpha chains of type I and type III procollagens are believed to be essential for correct chain recognition and chain assembly in these molecules. We studied here whether the 30-kDa C-propeptides of the human pCalpha1(I), pCalpha2(I), and pCalpha1(III) chains, i.e. proalpha chains lacking their N-propeptides, can be replaced by foldon, a 29-amino acid sequence normally located at the C terminus of the polypeptide chains in the bacteriophage T4 fibritin. The alphafoldon chains were expressed in Pichia pastoris cells that also expressed the two types of subunit of human prolyl 4-hydroxylase; the foldon domain was subsequently removed by pepsin treatment, which also digests non-triple helical collagen chains, whereas triple helical collagen molecules are resistant to it. The foldon domain was found to be very effective in chain assembly, as expression of the alpha1(I) foldon or alpha1(III) foldon chains gave about 2.5-3-fold the amount of pepsin-resistant type I or type III collagen homotrimers relative to those obtained using the authentic C-propeptides. In contrast, expression of chains with no oligomerization domain led to very low levels of pepsin-resistant molecules. Expression of alpha2(I)foldon chains gave no pepsin-resistant molecules at all, indicating that in addition to control at the level of the C-propeptide other restrictions at the level of the collagen domain exist that prevent the formation of stable [alpha2(I)](3) molecules. Co-expression of alpha1(I) foldon and alpha2(I) foldon chains led to an efficient assembly of heterotrimeric molecules, their amounts being about 2-fold those obtained with the authentic C-propeptides and the alpha1(I) to alpha2(I) ratio being 1.91+/-0.31 (S.D.). As the foldon sequence contains no information for chain recognition, our data indicate that chain assembly is influenced not only by the C-terminal oligomerization domain but also by determinants present in the alpha chain domains.