--Alan L. Kriz, Faith C. Belanger and Cheryl A. Green
The maize Glb1 gene, which encodes a vicilin-like embryo storage
protein, is highly polymorphic, and several naturally occurring alleles
have been described. The most common alleles, referred to as size alleles,
encode mature polypeptides ranging in molecular mass from about Mr 60,000
to Mr 70,000, as determined by SDS-PAGE. We recently reported the nucleotide
sequences of the Glb1-L (Large protein) and Glb1-S
(Small protein) alleles (Genetics 129:863, 1991) and determined
that the size difference between the two proteins encoded by these alleles
is due to a 12 amino acid duplication in the GLB1-L protein relative to
the GLB1-S protein. To further investigate the nature of allelic variation
of Glb1, we cloned and sequenced the Glb1-V allele, the protein
product of which has been reported by Osterman (Biochem. Genet. 26:463,
1988) to differ from the L and S proteins in the manner of
protein processing. The sequence of Glb1-V is very similar to that
of the other Glb1 alleles, except that deletion of a single residue
in the last (fifth) exon results in a frameshift mutation which dramatically
alters the characteristics of the protein, as determined by deduction of
the amino acid sequence from the nucleotide sequence. The most notable
difference between the GLB1-V protein and the other GLB1 proteins is an
effective substitution of several glutamic acid residues in the carboxy-terminal
third of the polypeptide with arginine residues, due to the change in reading
frame. The net result of these acidic to basic amino acid substitutions
is a drastic change in the isoelectric point of the protein from about
6.5 to 12.3. Some characteristics of the different allelic forms of mature
Glb1 proteins, as deduced from nucleotide sequence information,
are as follows:
| GLB1-L | GLB1-S | GLB1-V | |
| mol. wt. | 56,300 | 55,100 | 53,500 |
| pI | 6.2 | 6.8 | 12.3 |
| # arg residues | 61 | 59 | 83 |
| # basic aa, total | 100 | 98 | 115 |
| # acidic aa, total | 88 | 82 | 37 |
It is not apparent from the primary sequence of the GLB1-V protein how the processing of this polypeptide would differ from that of other GLB1 proteins. It is likely that the sequence changes introduced by the frameshift mutation alter the structure of the protein in such a manner as to result in altered processing. Analysis of GLB1-V protein synthesis may provide insight to the nature of globulin processing in maize embryos.
It is also interesting to note that the introduction of a large number
of basic amino acid residues does not affect accumulation of the GLB1-V
protein in maize embryos. One of the objectives of our laboratory is to
use modified versions of the Glb1 gene, engineered to contain a
large number of lysine codons, as a means of enhancing the nutritional
quality of maize grain protein. Mother Maize has shown us that the seed
is capable of tolerating a Glb1-encoded protein with a large number
of basic arginine residues, and it is likely that modified polypeptides,
containing large numbers of basic lysine residues rather than arginines,
will effectively be accumulated in the seed as well.
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