The COBRA Family of Putative GPI-Anchored Proteins in Arabidopsis. A New Fellowship in Expansion

Abstract

Identification of regulatory molecules that determine the extent and direction of expansion is necessary to understand how cell morphogenesis is controlled in plants. We recently identified COB (COBRA) as a key regulator of the orientation of cell expansion in the root. Analysis of the Arabidopsis genome sequence indicated that COB belongs to a multigene family consisting of 12 members, all predicted to encode glycosylphosphatidylinositol-anchored proteins. All but two of the COBL (COB-like) genes are expressed in most organs examined, suggesting possible redundancy. Sequence comparisons, phylogenetic analyses, and exon-intron positions revealed that the COB family is composed of two main subgroups sharing a common architecture, one subgroup being characterized by an additional N-terminal domain. Identification of expressed sequence tags corresponding to potential orthologs in other plant species suggested that COB-related functions are required in all vascular plants. Together, these results indicate that COB family members are likely to be important new players at the plasma membrane-cell wall interface.Cell morphogenesis is heavily influenced and restricted by the presence of an extracellular network of carbohydrates and proteins, the cell wall. Far from being an inert and stable exoskeleton, the primary wall is a highly dynamic extracellular matrix characterized by plastic, elastic, and viscous physical properties. These properties are conferred by the nature of the different constituent polymers, a load-bearing cellulose/xyloglucan (hemicellulose) array, and a compression-resistant pectin gel (Roberts, 1994). The developmental regulation of cell wall dynamics required for cell expansion and cell shape modeling must be directed by proteins capable of organizing, loosening, and rearranging the different polysaccharide networks and incorporating newly synthesized material. More than 17% of Arabidopsis genes contain a signal peptide sequence and over 400 proteins are annotated as being localized in the cell wall (Arabidopsis Genome Initiative [AGI], 2000), suggesting the possibility that more than 1,000 genes are implicated in wall biogenesis and modification .Proteins localized to the cell wall, such as the Hyp-, Pro-, or Gly-rich proteins, arabinogalactan proteins, and expansins, were originally identified through assays for biochemical and/or biophysical activities. More recently, forward and reverse genetic approaches have led to the identification of new classes of cell wall modifiers such as cellulose synthases (Arioli et al., 1998; Fagard et al., 2000), endo-␤-1,4-glucanases (Nicol et al., 1998), and wall-associated kinases (Kohorn, 2001). By analyzing Arabidopsis mutants exhibiting abnormal cell expansion during root development (Benfey et al., 1993;Hauser et al., 1995), COBRA was identified as an essential player in the regulation of the orientation of cell expansion (Schindelman et al., 2001). Reduced levels of crystalline cellulose microfibrils in the mutant suggested a role for COBRA in cellulose de...

Glycosylphosphatidylinositol-Anchored Proteins Are Required for Cell Wall Synthesis and Morphogenesis in Arabidopsis

“…We found that COB was undetectable in embryo extracts but present in callus tissue, where levels of COB protein are strongly reduced in pnt1 compared with the wild type. COB is one of an 11-member gene family, several of which are expressed during embryogenesis, and all of which have been either predicted or shown to be GPI anchored (Roudier et al, 2002;Borner et al, 2003). The magnitude of cellulose decrease in cob roots (;30%) is similar to that seen in pnt1 embryos (40%).…”

Molecular characterization, expression pattern, and function analysis of the OsBC1L family in rice

“…In addition, OsBC1L1, OsBC1L5, and OsBC1L8 were characterized by an additional N-terminal region of 170 amino acids (Fig. 1b), which was also found in the Arabidopsis AtCOBL7 subgroup and maize ZmBk2L1 subgroup (Roudier et al 2002;Brady et al 2007). …”

“…The COBRA gene encodes a glycosylphosphatidylinositol (GPI)-anchored protein that has an N-terminal signal sequence for secretion, a hydrophilic middle, a highly hydrophobic C terminus, a CCVS (Cys-rich) motif, an x-attachment site for GPI processing, two putative N-glycosylation sites, and two predicted cellulose binding sites (Roudier et al 2002). COBRA belongs to a multigene superfamily (COBRA-like family), a large protein family only existing in plants (Roudier et al 2002). There are 12 members in Arabidopsis (COBRA, AtCOBL1-11), 11 in rice (BC1, OsBC1Lp1, OsBC1L1-9), and 9 in maize (ZmBK2, ZmBK2L1, ZmBK2L3-9; Roudier et al 2002;Li et al 2003;Brady et al 2007).…”

“…COBRA belongs to a multigene superfamily (COBRA-like family), a large protein family only existing in plants (Roudier et al 2002). There are 12 members in Arabidopsis (COBRA, AtCOBL1-11), 11 in rice (BC1, OsBC1Lp1, OsBC1L1-9), and 9 in maize (ZmBK2, ZmBK2L1, ZmBK2L3-9; Roudier et al 2002;Li et al 2003;Brady et al 2007). Members of COBRA-like family are involved in diverse biological processes related to cell wall biosynthesis and cell expansion.…”

Stress Response Versus Stress Tolerance: A Transcriptome Analysis of Two Rice Lines Contrasting in Tolerance to Phosphorus Deficiency

“…Extensins and GPI-anchored proteins were not regulated in roots, and only one out of 34 cellulose synthase-related proteins responded to P deficiency in roots, that gene being downregulated. The cellulose synthase-like D1 (Os10g42750), shown to be required for root hair morphogenesis in rice (Kim et al 2007), was not affected by E or G. Based on studies with root-hair-defective Arabidopsis mutants, several additional genes have been linked to root hair development: guanosine triphosphatase (GTPase), glycerophosphoryl-diester-phosphodiesterase-like GPI-anchored protein, and the COBRA-like gene COBL9 (Roudier et al 2002;Hochholdinger et al 2008). In our study, these genes were not differentially regulated by factors E or G.…”