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. 1997 May;114(1):345–352. doi: 10.1104/pp.114.1.345

A defective signal peptide tethers the floury-2 zein to the endoplasmic reticulum membrane.

J W Gillikin 1, F Zhang 1, C E Coleman 1, H W Bass 1, B A Larkins 1, R S Boston 1
PMCID: PMC158310  PMID: 9159955

Abstract

The maize (Zea mays L.) floury-2 (fl2) mutation is associated with a general decrease in storage protein synthesis, altered protein body morphology, and the synthesis of a novel 24-kD alpha-zein storage protein. Unlike storage proteins in normal kernels and the majority of storage proteins in fl2 kernels, the 24-kD alpha-zein contains a signal peptide that would normally be removed during protein synthesis and processing. The expected processing site of this alpha-zein reveals a putative mutation alanine-->valine (Ala-->Val) that is not found at other junctions between signal sequences and mature proteins. To investigate the impact of such a mutation on signal peptide cleavage, we have assayed the 24-kD fl2 alpha-zein in a co-translational processing system in vitro. Translation of RNA from fl2 kernels or synthetic RNA encoding the fl2 alpha-zein in the presence of microsomes yielded a 24-kD polypeptide. A normal signal peptide sequence, generated by site-directed mutagenesis, restored the capacity of the RNA to direct synthesis of a properly processed protein in a cell-free system. Both the fl2 alpha-zein and the fl2 alpha-zein (Val-->Ala) were translocated into the lumen of the endoplasmic reticulum. The processed fl2 alpha-zein (Val-->Ala) was localized in the soluble portion of the microsomes, whereas the fl2 alpha-zein co-fractionated with the microsomal membranes. By remaining anchored to protein body membranes during endosperm maturation, the fl2 zein may thus constrain storage protein packing and perturb protein body morphology.

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Selected References

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