Expression and localization of AtAGP18, a lysine-rich arabinogalactan-protein in Arabidopsis.

Arabinogalactan-proteins (AGPs) are present on the surface of all plant cells. AtAGP17, 18 and 19 comprise the lysine-rich AGP subfamily in Arabidopsis and consist of an N-terminal signal peptide, a classical AGP domain interrupted by a small Lys-rich region and a C-terminal glycosylphosphatidylinositol (GPI) anchor addition sequence. Organ- and tissue-specific expression patterns and subcellular localization of AtAGP18 were studied and compared to other Lys-rich AGPs. AtAGP18 was highly expressed in roots, flowers and stems and weakly expressed in seedlings and rosettes. High AtAGP18 promoter activity was closely associated with vascular tissues and high in young organs as well as styles. Microarray and massively parallel signature sequencing (MPSS) data were also examined and showed largely consistent transcription profiles of AtAGP18. On the protein level, AtAGP18 was most abundant in roots and flowers, moderate in stems, seedlings and siliques and low in rosette leaves. Furthermore, AtAGP18 was localized to the plasma membrane and to Hechtian strands, following plasmolysis of tobacco cultured cells expressing a green fluorescence protein (GFP)-AtAGP18 fusion protein. Localization of AtAGP18 on the plasma membrane was further confirmed by biochemical two-phase fractionation and Western blotting. These expression and localization data further our understanding of AtAGP18 and provide a molecular basis to approach and decipher its function.