Extensin-Like Glycoproteins in the Maize Pollen Tube Wall

Abstract

We recently described the cloning and characterization of Pex1, a maize pollen-specific gene with an extensin-like domain. Here, we report that antibodies raised against a Pex fusion protein and a Pex synthetic peptide recognize a protein doublet with an apparent molecular mass of ~300 kD as well as larger proteins in pollen extracts. These proteins were not detected in extracts of seedling, endosperm, ear, silk, root, leaf, wounded leaf, meiotic tassel, or young microspore. After deglycosylation, only the protein doublet was detected by the anti-Pex antiserum, suggesting that the higher molecular mass proteins represent a glycosylated form of the Pex proteins. The anti-Pex antiserum was also used in immunolocalization experiments with in vitro-germinated pollen. With the aid of a confocal light microscope, the Pex proteins were localized to the pollen tube wall. The Pex proteins could not be removed with high salt, SDS, or chaotropic or reducing agents, suggesting a very tight association with the pollen tube wall. Immunocytochemical analysis at the ultrastructural level localized the Pex proteins to the intine in mature pollen and to the callosic sheath of the pollen tube wall in germinated pollen. Localization to the pollen tube wall strongly suggests that the Pex proteins play a role in pollen tube growth during pollination.

Whole-Genome Comparison of Leucine-Rich Repeat Extensins in Arabidopsis and Rice. A Conserved Family of Cell Wall Proteins Form a Vegetative and a Reproductive Clade,

“…The LRX1 protein is specifically targeted to the root hair cell wall, where it is insolubilized. Similarly, PEX1, a pollen-specific maize (Zea mays) LRX, was immunolocalized in the intine layer of the pollen grain and the callosic sheath of the pollen tube (Rubinstein et al, 1995a(Rubinstein et al, , 1995b. Together, the properties of LRR and extensins, the cell wall localization of LRX proteins, and the phenotype of lrx1 mutants suggest that LRXs are potentially involved in the regulation of cell wall expansion in response to signaling.…”

“…The LRX genes are dispersed on the five Arabidopsis chromosomes and on seven rice chromosomes. The nomenclature used to identify the individual genes has been developed based on previous work (Rubinstein et al, 1995a(Rubinstein et al, , 1995bBaumberger et al, 2001; this study). The gene family is referred to as the LRX gene family.…”

“…It is interesting to note that none of the LRXs contains the motif VYK, thought to be important for intermolecular crosslinking (Schnabelrauch et al, 1996). Because protein immobilization in the cell wall was demonstrated for AtLRX1 and ZmPEX1 (Baumberger et al, 2001;Rubinstein et al, 1995b), it is likely that this process is mediated by other motifs than those so far identified in extensins.…”

“…Therefore, it is particularly intriguing to note that at least four Arabidopsis LRX genes are expressed in mature and possibly in germinating pollen as demonstrated for the ZmPEX1 gene (Rubinstein et al, 1995a(Rubinstein et al, , 1995b. Thus, the primary function of LRX proteins might be to control cell polarization or to locally regulate cell wall expansion during tip growth.…”

RALFs

“…Another possible class of RALF-binding proteins is the cell wall localized Leucine-rich Repeat eXtensin chimera (LRX) proteins. [30][31][32][33] When a tomato pollen-specific LRX recognition domain was used as a bait in a Yeast 2 Hybrid screen, a pollen specific RALF was identified. 18 While it is intriguing to note that several plant peptide receptors bind to their ligands via Leucine-rich Repeats (LRRs), the relationship of the RALFbinding proteins identified by Scheer et al 29 and the cell wall localized LRX proteins is not yet known.…”