The presence of acidic amino acids, most frequently an aspartic or glutamic acid, is the key feature of tyrosine sulfation sites (Lee & Huttner 1983). Sulfation is a general characteristic of type III procollagen, into which the anion is incorporated in the form of tyrosine-O-sulfate (Jukkola et al. 1986). Interestingly, inhibition of tyrosine sulfation does not seem to affect the secretion or processing of type III procollagen in in vitro experiments (Jukkola et al. 1990). Since it has been suggested that charged and hydrophobic amino acids could contribute to the prevention of premature fibril formation and the regulation of collagen fiber diameter (Fleischmajer & Perlish 1986), one possible function of tyrosine sulfation lies in collagen fibrillogenesis. Furthermore, these sulfate residues, due to their negative charge, could influence the interactions of type III procollagen with other extracellular matrix components.
