Composition and synthesis of the pectin and protein components of the cellwall of Closterium acerosum (Chlorophyta)

Closterium acerosum Ehrenberg (Chlorophyta) possesses a trilayered cell wal l consisting of an outer trilaminate stratum, a fibrous middle layer, and a thick inner fibrous layer. The outermost layer has a series of external pa rallel ridges and valleys. At the bases of the valleys are the wall pores, the site of mucilage release. Pure fractions of cell walls were isolated an d inclusive pectin and wall protein fractions were extracted and characteri zed. Two pectin-like fractions were isolated: a CDTA-extracted polymer cons isting of 60.1% galacturonic acid and a Na2CO3-extracted fraction consistin g of 39.9% galacturonic acid. Two major protein fractions, one with a molec ular mass of 23.5 kDa and one with a molecular mass of 28.5 kDa, were isola ted by preparative gel electrophoresis. The former was glycine-rich, wherea s the latter contained both significant amounts of glycine and hydroxyproli ne. Antibodies were raised to both the pectin fractions and the 23.5-kDa wa ll protein fraction. Immunocytochemical labeling of whole cells and wall fr agments using antibodies raised against CDTA and Na2CO3 extracts showed tha t these pectin-like components were found throughout the wall strata and we re more concentrated at the polar tips, the site of new wall synthesis in g rowing semicells. Immunogold labeling showed that their production was focu sed on the trans Golgi network of the Golgi apparatus. Immunolabeling with an antibody raised against the 23.5-kDa glycine-rich wall protein showed cl ose association of the protein with the wall pores. Similarly, immunogold l abeling revealed that the protein was processed throughout the entire Golgi body even when large mucilage-containing vesicles were being processed. Th e roles of the secretory apparatus and putative spitzenkorper-like regions of the cell are discussed.