Sequence and expression of a gene encoding an albumin storage protein in sunflower

The complete sequence of a sunflower (Helianthus annuus) gene, HaG5, encoding a 2 S albumin storage protein was determined. The predicted unprocessed precursor has 295 amino acids, is rich in glutamine residues (24%) and contains a hydrophobic amino-terminus that is similar to the consensus signal peptide. Amino acid sequencing of the mature protein revealed extensive post-translational processing. Nuclease protection and primer extension analysis indicated a major transcriptional start 30 nucleotides 5' of the predicted ATG start codon. Additional sequence data, determined from a nearly full length cDNA recombinant, indicate that HaG5 is a member of a small gene family comprised of at least two divergent genes. Comparison of the predicted HaG5 gene product with sequences of other known plant proteins revealed distant but significant homology with the napins of Brassica and other heterogeneous seed proteins in the albumin superfamily.