Relationships between Hydroxyproline-containing Proteins Secreted into the Cell Wall and Medium by...

The pathway of hydroxyproline-containing proteins to the cell wall, and to the growth medium in suspension-cultured Acer pseudoplatanus cells is traced by following the kinetics of the transfer of protein-bound (14)C-hydroxyproline into various fractions, and by comparing the hydroxyproline-arabinoside profiles of these fractions after alkaline hydrolysis.Hydroxyproline-rich protein passes directly from a membrane-bound compartment in the cytoplasm to the cell wall, not via an intermediate salt-soluble pool in the wall.There are at least three hydroxyproline-containing glycoproteins in the cell wall. One which possesses mono, tri, and tetraarabinoside side chains accounts for over 90% of the total hydroxyproline. This glycoprotein is "extensin."The hydroxyproline-containing proteins secreted into the medium have a glycosylation pattern markedly different from that of the major cell wall glycoprotein. It appears that there is little or no wall-like extensin in the medium.Approximately half of the protein-bound hydroxyproline secreted into the medium is linked to an arabinogalactan. This linkage is also found in a particulate wall protein precursor fraction from the cytoplasm, but only trace amounts can be detected in the cell wall.