Cloning and characterization of a pollen-specific cDNA encoding a glutamic-acid-rich protein (GARP) from potato Solanum berthaultii

A pollen-specific cDNA was isolated from a cDNA library of in vitro germinated pollen of the diploid potato species Solanum berthaultii. The cDNA clone, designated SB401, hybridizes to a messenger RNA of 1.2 kb length in mature and germinated pollen. SB401 messenger RNA is absent from other parts of the plant, including other flower tissues. SB401 cDNA, which possesses a long stretch of AT-rich 5'-untranslated leader sequence, encodes a glutamic acid-rich protein (GARP) which is hydrophilic throughout and contains six imperfect repeated motifs of the sequence V-V-E-K-K-N/E-E with the di-basic amino acid residue pair (K-K) as the core within the repeats. These repeats are spaced at irregular intervals and predicted to form an alpha-helical structure. The SB401 protein was over-expressed in Escherichia coli and the purified protein was used for raising antiserum. Both E. coli-expressed and the endogenous SB401 proteins in pollen and pollen tubes appear much larger on SDS-polyacrylamide gels than their calculated molecular masses. Immunoblotting revealed the protein to be most abundant in germinated pollen. The structural features of SB401 protein and a possible role for the protein in pollen development, pollen germination, and pollen tube growth are discussed.