DcAGP1, a secreted arabinogalactan protein, is related to a family of basic proline-rich proteins

A cDNA corresponding to the core protein of an immunoaffinity-purified arab inogalactan protein (AGP) secreted by Daucus carota (carrot) cells in liqui d culture was isolated. This cDNA, DcAGP1, encodes a new class of `non-clas sical' AGP with strong similarity to a family of basic proline-rich protein s. The protein is rich in proline (17%), alanine (10%) and lysine (11%) and contains four distinct domains: a signal peptide, a proline-rich domain, a histidine-rich basic domain and a cysteine-containing 'PAC' domain that is found in a range of other cell wall proteins. The protein contains several sequence motifs found in otherwise unrelated cell wall proteins, but also displays some unique features. Northern blot analyses show that while the D cAGP1 transcript is abundant in the suspension-culture cells from which the AGP was obtained; in carrot seedlings the gene is only expressed at low le vels in the roots and it is neither wound- nor stress-inducible. Furthermor e, northern and western blot analyses demonstrate that the core polypeptide of DcAGP1 is differentially glycosylated in two different carrot suspensio n cultures. The unusual features of the protein sequence suggest that the D cAGP1 protein is a member of a family of basic proline-rich proteins define d by the C-terminal PAC domain, and the possible function(s) of the DcAGP1 protein is considered in the light of current views on AGP structure and fu nction.