Glycosylated polyproline II rods with kinks as a structural motif in planthydroxyproline-rich glycoproteins

Hydroxyproline-rich glycoproteins (HRGPs) are the major proteinaceous compo nents of higher plant walls and the predominant components of the cell wall of the green alga Chlamydomonas reinhardtii. The GPI protein, an HRGP of t he C. reinhardtii wall, is shown to adopt a polyproline II helical configur ation and to carry a complex array of arabinogalactoside residues, many bra nched, which are necessary to stabilize the helical conformation. The deduc ed GP1 amino acid sequence displays two Ser-Pro-rich domains, one with a re peating (SP), motif and the other with a repeating (PPSPX)(x) motif. A seco nd cloned gene a2 also carries the PPSPX repeat, defining a novel gene fami ly in this lineage. The SP-repeat domains of GP1 form a 100-nm shaft with a flexible kink 28 nm from the head. The gp1 gene encodes a PPPPPRPPFPANTPM sequence at the calculated kink position, generating the proposal that this insert interrupts the PPII helix, with the resultant kink exposing amino a cids necessary for GP1 to bind to partner molecules. It is proposed that si milar kinks in the higher plant HRGPs called extensins may play a comparabl e role in wall assembly.