Earlier we isolated a threonine-rich extensin from maize (Zea mays). Here, we report that maize cell suspension cultures yield a new extensin rich in histidine (HHRGP) that also has characteristics of arabinogalactan proteins (AGPs). Thus, chymotryptic peptide maps of anhydrous hydrogen fluoride (HF)-deglycosylated HHRGP showed repetitive motifs related to both extensins and AGPs as follows. HHRGP contains Ala-Hyp3 and Ala-Hyp4 repeats that may be related to the classical dicot Ser-Hyp4 extensin motif by the single T -* G (Ser --Ala) base change. Furthermore, HHRGP also contains the repetitive motif Ala-Hyp-Hyp-HypHis-Phe-Pro-Ser-Hyp-Hyp related to the Ser-Hyp4-Ser-Hyp-SerHyp4 motif of P3-type dicot extensin. However, HHRGP also has AGP characteristics, notably an elevated alanine content, near sequence identity with the known Lo/ium AGP peptide Ser-HypHyp-Ala-Pro-Ala-Pro, the putative presence of glucuronoarabinogalactan, and precipitation by Yariv antigen, but fl-elimination of arabinogalactan indicates its 0-linkage to serine rather than the characteristic 0-hydroxyproline link of other AGPs. Although HHRGP might be a "chimera" of two different proteins, i.e. an extensin and an AGP, this is unlikely because one can account for the apparent chimera by the codon relationships of the five common hydroxyproline-rich glycoprotein amino acid residues, Ser, Pro, Thr, Ala (TCx, CCx, ACx, GCx) and histidine (CAT or CAC), which facilitate interconversion of major motifs by single point mutations. Thus, we propose that the extensin family of wall proteins consists of a highly diversified phylogenetic series ranging from basic minimally glycosylated repetitive pro-rich proteins to the highly glycosylated acidic AGPs. To relate this diversity of form and function at the molecular level, we identified putative functional domains hypothetically involved in properties such as reptation, recognition, adhesion, intermolecular cross-linkage, and self-assembly. herbaceous dicots whose walls are often rich in extensins: rodlike HRGPs,2 generally highly basic and consisting of variably glycosylated repetitive peptide motifs (8, 13-18, 30, 31, 34 Recently, we characterized a graminaceous monocot HRGP homologous with dicot extensins (12, 14, 15) but with some significant modifications. Here, we report the isolation of a second HRGP from the graminaceous monocot, Zea mays. This HRGP is unusually rich in His, hence an HHRGP, and is an apparent extensin AGP "chimera," i.e. it shares sequence homology and glycosylation patterns with both extensins (17,18,30,31,34) and AGPs (7,9,24). However, we suggest that HHRGP is not a chimera oftwo different proteins per se but, instead, occupies an intermediate position between extremes of the extensin family whose members are a phylogenetic series ranging from highly repetitive basic minimally glycosylated RPRPs to the highly glycosylated acidic AGPs.
MATERIALS AND METHODS Suspension CulturesWe grew maize (Zea mays) cell suspensions (cv Black Mexican) in 1-L flasks containing 500 mL Muras...