The major proteins specific to maize embryos have been identified as the products of the Pro locus on chromosome 1 (MGG 174:233-240, 1979)(this locus will be referred to as Prot in accordance with its current designation by the Maize Genetics Coop). This locus codes for two proteins, PROT' and PROT; PROT' is about 5,000 daltons larger than PROT (three size alleles, Prot-L, Prot-I and Prot-S, with molecular weights ranging from about 61,000 to 68,000 daltons, have been reported). These proteins fit the criteria of storage proteins as defined by Derbyshire et al. (Phytochemistry 15:3-24, 1976): they are first detected (by SDS-polyacrylamide gel electrophoresis) about 20 days post-pollination, they account for up to 30% of total protein in mature embryos, and they are rapidly degraded on germination. These proteins are classified as globulins by virtue of their solubility in 1M NaCl and insolubility in water, and by precipitation at pH 4.7. These characteristics also suggest that the products of the Prot locus comprise the "embryo antigen" P7 of Khavkin et al. (Planta 143:11-20, 1979).
Extensive homology between PROT' and PROT is observed by digestion with trypsin, chymotrypsin, S. aureus V8 protease, formic acid, and cyanogen bromide; the proteins specified by the three size alleles also show considerable homology in peptide digests. The cyanogen bromide digests have been found to be quite informative when comparing the size alleles. Cyanogen bromide cleaves the proteins into two major fragments: the size difference between PROT' and PROT is limited to the larger fragment, whereas the size differences observed between the proteins specified by the different size alleles are limited to the smaller fragment. Experiments are in progress to determine which fragment represents the amino-terminus of the protein.
PROT' is first detected in SDS gels about 20 days post-pollination, and PROT first appears two to three days later. The unlinked locus Mep (chromosome 5) controls the synthesis of PROT. In homozygous mep embryos, very little PROT is synthesized, and PROT' accumulates. The action of Mep does not appear to involve protein processing since preliminary in vivo experiments have failed to show a chase of radioactively-labeled amino acids from PROT' to PROT in immature embryos pulsed with label. It appears that the action of Mep is at the level of transcription or RNA processing.
Al Kriz and Drew Schwartz
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