Extensin chimeras in the pollen extracellular matrix: Possible role in cell-cell communication

We have identified a family of pollen glycoproteins, called Per proteins (P ($) under bar ollen ex ($$) under bar tensin-like). The Per proteins are extensin chimeras, with a C-terminal domain having numerous copies of the Ser-Pro, extensin motif and an N-terminal domain that includes Leucine-rich repeats (LRRs), a motif that mediates protein-protein interactions. Maize (mPex) and tomato (tPex) Per genes encode both domains and are expressed only in pollen. Only one 20-30 aa segment of the tomato and maize proteins (as well as candidate Per proteins from Arabidopsis, sorghum and potato) is highly variable when the protein sequences are aligned. This hypervariable region is adjacent to the LRRs of the Per proteins. The extensin domains of the Per proteins are not highly conserved at the sequence level, but are all likely to have a rod-like polyproline II configuration. The 12 amino acids at the carboxy terminus of all of the Per proteins are highly conserved, with two tyrosine residues that may be involved in anchoring the protein in the pollen tube cell wall. Antisense experiments using transgenic tomato have been conducted to further investigate the function of these proteins. In several transgenic lines, the antisense tPex gene is not transmitted through the pollen, consistent with a requirement for functional Per protein in pollen during reproduction. Antisense tPex gene containing pollen is morphologically normal and is able to hydrate and germinate a pollen tube in vitro and in vivo. We are now examining growth of antisense tPex pollen within the stigma, style and ovary of pistils. Both the structural aspects of the Per proteins and the phenotype of antisense expressing plants are suggestive of a recognition role for the Per proteins.