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In vitro Uptake and Processing of Prezein and Other Maize Preproteins by Maize Membranes

F. A. Burr and B. Burr
The Journal of Cell Biology
Vol. 90, No. 2 (Aug., 1981), pp. 427-434
Stable URL: http://www.jstor.org/stable/1609484
Page Count: 8
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In vitro Uptake and Processing of Prezein and Other Maize Preproteins by Maize Membranes
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Abstract

A cell-free, mRNA-dependent system has been developed for the translation and processing of zein preproteins. A rough endoplasmic reticulum (RER)-enriched fraction, isolated by sucrose density gradients, can be treated with micrococcal nuclease to destroy endogenous messages. When these membranes are added to a wheat germ protein-synthesizing system together with zein mRNA, synthesis and processing of the polypeptides to the mature products takes place. The RER fraction from the endosperm has a different protein composition than that prepared from either the shoot or nucellar tissue and processes prezein more efficiently. The cleavage of the preproteins appears to be a cotranslational step as the completed preprotein chains cannot be processed, although they can be taken up to a limited extent. This small uptake, or absorption, of unprocessed zein seems to be an artifact and may be related to the unusual solubility properties of zein. Finally a sodium dodecyl sulfate (SDS)-urea polyacrylamide gel system has been developed which is particularly suited for the separation of low molecular weight proteins (<10,000 daltons). Using this method, we examined the products of in vitro zein processing and detected no presequence polypeptides. This suggests that the zein cleavage proteinase is probably an exopeptidase.

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