A leucine-rich repeat region is conserved in pollen extensin-like (Pex) proteins in monocots and dicots

We previously isolated a pollen-specific gene encoding a pollen tube wall-associated glycoprotein with a globular domain and an extensin domain from maize (mPex1). To evaluate which protein domains might be important for function, we isolated a second monocot gene (mPex2) and a dicot gene (tPex). Each gene encodes a signal sequence, an N-terminal globular domain comprised of a variable region, a leucine-rich repeat (LRR) with an adjacent cysteine-rich region, a transition region and an extensin-like C-terminal domain. The LRRs of the maize and tomato Pex proteins are highly conserved. Although the extensin domains in the maize and tomato proteins vary in length and in amino acid sequence, they are likely to be structurally conserved. Additional putative Pex gene sequences were identified by either GenBank search (Arabidopsis) or PCR (sorghum and potato); all encode conserved LRRs. The presence of a conserved LRR in the known and potential Pex proteins strongly suggests that this motif is involved in the binding of a specific ligand during pollen tube growth. Gene expression studies using RNA and protein blotting as well as promoter-reporter gene fusions in transient and stable transformation indicate that the tomato Pex gene is pollen-specific.