Preparation and chemical characterization of cell wall fractions enriched in structural proteins from Palmaria palmata (Rhodophyta)

A protocol using physical, chemical and enzymatic treatments was developed to prepare cell wall fractions enriched in structural proteins from the red alga Palmaria palmata. The walls obtained from cortical and medullary cells of soft and rigid thalli were chemically characterized. They were all rich in proteins (2936%(DW)) and contained ash (434%(DW)), xylose (619%(DW)), glucose (513%(DW)) and galactose (1%(DW)). Fractions from cortical cells were richer in ash, while those from medullary cells were richer in xylose. The amino acid composition of the fractions differed from that of the initial starting algal materials.They were enriched in tyrosine and methionine, which could indicate peculiar protein crosslinks such as phenolic couplings and/or disulfur bridges. Extraction of these fractions by 2 M NaCl isolated minor amounts of materials (approximately 10%(DW) of cell wall fractions) containing glucose, xylose, proteins, galactose and sulfates. The proteinrich insoluble residues were partially degraded by bromelain, papain, trypsin and chymotrypsin into minor amounts of proteic fractions, indicating that the proteases have a restricted access to the cell wall proteins, probably as a result of high proportions of crosslinks. Some fragments contained proteins and xylose, galactose and glucose, suggesting the existence of glycoside sidechains on proteins and/or linkages between cell wall polysaccharides and proteins. The difference in the composition and structure of proteins in the cell wall fractions from soft vs. rigid thalli, and from cortical vs. medullary cells demonstrated that these cell wall structures are closely regulated in relation to cell development and specialization.