Synthesis of an unusual α-zein protein is correlated with the phenotypic effects of the floury2 mutation in maize

Abstract

The soft, starchy endosperm of the maize (Zea mays L.) floury 2 mutant is associated with a reduction in zein mRNA and protein synthesis, unique protein body morphology, and enhanced levels of a 70 kDa protein, that has been shown to be the maize homolog of a chaperonin found in the endoplasmic reticulum. We found an unusual alpha-zein protein of 24 kDa to be consistently associated with the zein fraction from floury 2 mutants. Three additional alpha-zein proteins with molecular weights ranging from ca. 25 to 27 kDa are detected in the storage protein fraction of a high percentage of floury 2 kernels and a low percentage of normal kernels in a genetically segregating population. The four proteins in a genetically segregating population. The four proteins can be distinguished from one another by immunostaining on Western blots. Synthesis of the 24 kDa protein is regulated by Opaque2, since the 24 kDa protein is lacking in the storage protein fraction of opaque2/floury2 double mutants. The synthesis of an abnormal alpha-zein protein in floury2 could explain many features of the mutant, such as the abnormal protein body morphology, induction of the 70 kDa chaperonin, and hypostasis to opaque2 (o2). Although we cannot prove that the accumulation of this protein is responsible for the floury2 phenotype, we were able to detect a restriction fragment length polymorphism (RFLP) linked to the floury2 locus with a 22 kDa alpha-zein probe. We hypothesize that the unique characteristics of the floury2 mutant could be a response to the accumulation of a defective alpha-zein protein which impairs secretory protein synthesis.

Characterization of Maize Elongation Factor 1A and Its Relationship to Protein Quality in the Endosperm

“…Compared with the wild type, the protein bodies in 02 are much smaller, whereas in j72 they are both smaller and irregularly shaped, with convoluted morphology (Lending and Larkins, 1992;Lopes et al, 1994). We speculate that eEFlA is increased in these mutants as a consequence of increased protein body surface area and, consequently, an associated increase in the cytoskeletal network surrounding the rough ER.…”

“…Two naturally occurring maize mutants, opaque2 (02) and fzoury2 @2) were found to contain higher levels of Lys in the endosperm (Mertz et al, 1964;Nelson, et al, 1965). 02 encodes a transcription factor that regulates zein mRNA synthesis (Schmidt et al, 1990), whereas fz2 is a mutant a-zein, the abnormal processing of which affects storage protein synthesis and protein body formation (Lending and Larkins, 1992;Lopes et al, 1994;Coleman et al, 1995). We showed that the increase in Lys content in 02 coincides with a significant increase in eEFlA (Habben et al, 1993).…”

A highly digestible sorghum mutant cultivar exhibits a unique folded structure of endosperm protein bodies

“…In the naturally occurring maize floury2 mutant, protein bodies exhibit a unique morphology consisting of an amorphous structure (25). Lopes et al (25) found an unusual ␣-zein of 24 kDa in this mutant, and hypothesized that the unique features of floury2 may be a result of the accumulation of a defective ␣-zein outside of the endoplasmic reticulum.…”

A Defective Signal Peptide Tethers the floury-2 Zein to the Endoplasmic Reticulum Membrane

“…The extent of protein body deformation is positively correlated with the number of gene doses of thefl2 allele (Lending and Larkins, 1992). Furthermore, in fl2-opaque-2 double mutants, which do not synthesize the 24-kD a-zein, only small protein bodies were observed, and these maintained a normal, spherical shape (Lopes et al, 1994).…”