Relationships between Hydroxyproline-containing Proteins Secreted into the Cell Wall and Medium by Suspension-cultured Acer pseudoplatanus Cells

Abstract

The pathway of hydroxyproline-containing proteins to the cell wall, and to the growth medium in suspension-cultured A. pseudoplatanus cells is traced by following the kinetics of the transfer of protein-bound 14C-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 3 hydroxyproline-containing glycoproteins in the cell wall. One which possess 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 extension in the medium. Approximately half of 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.